To sleep in it would be inappropriate (5)
I believe the answer is:
inapt
'inappropriate' is the definition.
(I've seen this before)
'to sleep in it' is the wordplay.
'to sleep' becomes 'nap' (a nap is a short sleep).
'in' means one lot of letters goes inside another.
'nap' put into 'it' is 'INAPT'.
'would be' acts as a link.
(Other definitions for inapt that I've seen before include "Out of place" , "unqualified" , "Unsuitable - paint (anag)" , "Inappropriate" , "It's clumsy" .)