Another Ns Clue...

[Baldgrace]

I think I have the answer to this one, but why is it the answer?
Knowing about cut in extravagant documentary (3-2-3-4)

...I think it is "fly-on-the-wall" and I get the documentary reference, but not the rest! Any thoughts?
Thanks!

[bibblebub]

knowing = FLY
about ON
cut in extravagant = HEW in TALL = THEWALL

[Calibax]

Fly = knowing
on = about
hew =cut in Tall = extravagant

[Mamyalynne]

Knowing (FLY) about (ON)cut (HEW)in extravagant (TALL)documentary (Def)

[Lie-in King]

'Fly' (knowing) + 'on' (about) + 'hew' (cut, in) 'tall' (extravangant)

[Baldgrace]

Blimey, complicated. Thank you! I'd never have got there.