Size of paper (8)
I believe the answer is:
foolscap
'size of paper' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for foolscap that I've seen before include "About 330 x 200 (or 400) mm size of paper" , "One would not be wise to put this on" , "Paper or book size (from its watermark)" , "Cop's foal (anag)" , "something near A4" .)