Inappropriate forty winks in it (5)
I believe the answer is:
inapt
'inappropriate' is the definition.
(I've seen this before)
'forty winks in it' is the wordplay.
'forty winks' becomes 'nap' (synonyms).
'in' means one lot of letters goes inside another.
'nap' inserted into 'it' is 'INAPT'.
(Other definitions for inapt that I've seen before include "Far from fit" , "not fit for purpose" , "Inappropriate" , "Unsuitable - paint (anag)" , "Ill-chosen, not suitable" .)