1 is thematic - 'mag' is dialect for gossip/talk (you need a 7 letter word) and a 5 letter word for 'funds' - or the things they're collected in (a common word).
There are loads of oscure words in this puzzle, hankir - and the setter has made it harder with the extra letters. If you don't mind wading through it I'll carry on - or I can just give answers (I'll wait a few minutes before the next ones).
ok hankir - and yes, it's very British (but 17d is famous just about everywhere - '[the] ???? ????') and the last 4 ???? is part of all of the thematics - but not found in the grid.
7d is thematic - def in the style of a very famous Eng poet (ans. 9 letters) anag 'NO LIMIT[s]' + (mas)S (mediu)M - [s] is extra.
17d is thematic - DEN[t] reversed inside 3 letters for 'self confidence' (think psychology) also lifted - def = 'nervous'[t] is extra.
I'll see if i can help with the others a bit more...
21d is obscure for the def, but the female singer is the opera one (4 letters) and to punish students you 'GATE' them - the usual 2 letter 'popular' starts the answer and [c] is extra ([c]lose).
It's another thematic answer (10 letters) and this might be a bit too British for you, but she's famous for riding a horse through Coventry a long, long time ago.
I have 1a starting with C with X somewhere in the answer. 7 letters in cell 1.
7d is about poet John.
17d. Answer is 2, 4 with two letters going in cell 1.
21. New word for me but have the first 4 letters in cell 1.
Three clashes give me horse rider, mothers and nude.
I have not solved all clues, but the message is about poison.
Still working on 8a, 16a, 1d, 2d and 25d.
...yes, that all sounds good, hankir (sorry I misled with 'popular' which just forms part of the opera singer bit, and not 'in' as it usually does - anyway, it didn't hold you up).
1 I'm sure you know is a famous (notorious?) literary heroine.
For [there's no 8a?]6a the extra is 'cha[m]ps' and its thematic part makes it a 6 letter answer.
for 16a extra is 'die[t]' and its thematic part makes it a 5 letter answer (def - 'online', or even, 'sent down the line').
1d extra is 'da[r]ted' - thematic makes it 8 letters (take 'at'+S out of 'essentially' and anag).
2d 'boss' = slang 'mistake' (as is the answer) extra 'bo[a]'
25d thematic (archaic) def = 'remembers' - extra ('he[i]st' + in + bk for anag).
Thanks ProfessorM.
Please ignore mothers. I had placed clash in 17d/23a, now corrected to be in 25d.
Still working on 6a, 8d and 16a.
I have clashes in 1a/1d, 6a/7d, 16a/17d, 21a/21d, 23a/25d and 29a/28d.
What is the def in 8d.
I have the name of the movie.