Not appropriate to have forty winks in it (5)
I believe the answer is:
inapt
'not appropriate' is the definition.
(I've seen this before)
'forty winks in it' is the wordplay.
'forty winks' becomes 'nap' ('nap' can be a synonym of 'forty winks').
'in' means one lot of letters goes inside another.
'nap' going within 'it' is 'INAPT'.
'to have' acts as a link.
(Other definitions for inapt that I've seen before include "Not suitable or appropriate" , "Ill-suited" , "Cartoon duck" , "not fit for purpose" , "It's clumsy" .)