Inquisitor 1305

[Ripper]

I've filled in the grid (and spotted many references to the milestone) I've got the rhetorical question. But now I'm completely stuck!
"each of four answers describes a pair of others etc....."
I've just no idea how to progress
Any help gratefully appreciated!
Thanks in advance

[TheBear69]

Can someone explain the wordplay in 4A and 11A?

What's the definition for 4D? Seems like the clue consists of a charade + a homophone...

[kettledrum]

4a. Shield = something to protect. 'Eff' as in effing & blinding. Sheffield = city

[rabet]

Jagobean, may I ask how you know what the "milestone" is and where it was celebrated?

[Ornis]

11a. Nimrod's = Nimrod has = I've (he being the setter speaking). So you reverse (get back) alternations (intermezzi) of 'on' and 'Ive' to get envoi which means concluding section.

4d. MI6 = spies. Replace i (one) with ad(vertisement) = bill to get spades = spayads = obsolete word for deer (old dear, so to speak).