Side By Side By External ( The Crossword Centre )...

[Matakari]

Good morning, answers to the following would be kindly appreciated to see me through this one. Thanks in advance!
( www.crossword.org.uk/side.html )
INSTRUCTIONS:
Each row, with one exception, contains two letters omitted by wordplay in their clues. Cells containing these letters always occur in pairs, horizontally or vertically. These letters spell out a pair linked by a title spelled out in clue order by pairs of consecutive letters to be removed from eight other down clues before solving, always leaving real words. Finally, solvers must change one entry to show a further title linking the pair, creating new real-word crossing entries.
ACROSS:
16 River swimmer systematically crosses (6) : ??R???
21 Peers in Ayrshire pigpens (6) : ??I??T
25 Trump and English rogue not related (10) : ?N??A?????
27 Cockney has confidence and begins (5): ??E??
31 Opposing another player alone, no netballer tackles (8): ?H?????E
33 Forming into circle, soldiers stand in wine cellar (6): ?????E
DOWN:
5 Tricky to hide large excursion (5): ?ML??
28 Number of close encounters (5): ????N
24 Study dissecting slow climbing of sloths, maybe (7): ??????L
29 That is interrupted by upcoming race season (5): ????E

[Neveracrossword]

That's a pretty big batch, Matakari!

You have mistakes in 21, 31,5.

29 D Inure, season. Ie, with Run, race, inside (up, upcoming)
16 Mersey, in the clue, remembering the preamble.
28 Very similar to 28...
PS It's worth making the effort with this one, it turned out to be good fun.

[cashier]

5 sally?

[Neveracrossword]

Yes, it was was, Cashier. Sly, tricky, with L and an extra (as per the preamble).

[ProfessorMaisie]

I'm sure it's a typo in Neveracrossword's answer for 28 (28 is very similar to how 16 works).
21 is obscure dialect but 'pigpens' is the usual word ('m' is the missing letter).
25 - def is 'not related'/not told - anag TRUMP AND E (+ the missing 'i').
27 - drop the aitch from the beginning of 'has confidence'/trusts ('n' missing).
31 - also similar to how 16 works - and your 'h' is wrong (missing 'o')
33 the usual 2 letters for soldiers/rankers + BIN for the last 4 words.
24 - 4 letter word for 'slow' reversed around 3 letters for a study.

[Neveracrossword]

Thanks, PM - I was rushing!
Matakari, have we 'seen you through'? It looked as if you were a long way off!

[Matakari]

Thanks,NACW and ProfessorMaisie! I must confess that I am very poor at determining missing/extra letters in word play, etc… Could you give me an example of how the "m" and the "i" are arrived at in the following?
21 is obscure dialect but 'pigpens' is the usual word ('m' is the missing letter).
??I??T
25 - def is 'not related'/not told - anag TRUMP AND E (+ the missing 'i').
?N??A?????
For 22 Stretch ancient clasp (5) I have TACHE, but this seems to be wrong?
Thanks for your advice!

[Neveracrossword]

Matakari, you're looking at things the wrong way round. You don't 'arrive' at the 'omitted by wordplay' letter, that phrase means there's nothing in the wordplay for it! Basically you just solve the clue, with one unaccounted for letter. In 25 A, it's fairly straightforward - the 'rogue' indicates an anagram and you've listed the letters involved. The other one in the answer is the 'omitted by wordplay' one.
Tache is definitely wrong and there's a mistake in your 21.

[Matakari]

Many thanks, NACW!

[Neveracrossword]

Matakari, I hope you finished this off- it could have proved very useful later...