He cuts firm leaf (5)
I believe the answer is:
sheet
'leaf' is the definition.
(I've seen this in another clue)
'he cuts firm' is the wordplay.
'cuts' indicates putting letters inside (some letters cut their way into other letters).
'firm' becomes 'set' (both can mean fixed).
'he' put inside 'set' is 'SHEET'.
(Other definitions for sheet that I've seen before include "Piece of linen; ship's rope" , "Piece of paper or bedlinen" , "Set of unseparated stamps" , "Bed cover" , "Piece of linen, paper, etc" .)