Mostly nutty biscuit (7)
I believe the answer is:
cracker
'biscuit' is the definition.
(I've seen this before)
'mostly nutty' is the wordplay.
'mostly' means to remove the last letter.
'nutty' becomes 'crackers' ('crackers' can be a synonym of 'nutty').
'crackers' with its last letter taken away is 'CRACKER'.
(Other definitions for cracker that I've seen before include "pulled at Christmas?" , "See 8" , "Knockout biscuit?" , "Seasonal firework" , "regularly featured at Christmas" .)