Not appropriate to sleep in it (5)
I believe the answer is:
inapt
'not appropriate' is the definition.
(inapt means inappropriate or ill-fitting)
'sleep in it' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'in' indicates putting letters inside.
'nap' placed into 'it' is 'INAPT'.
'to' acts as a link.
(Other definitions for inapt that I've seen before include "Ill-suited" , "Cartoon duck" , "Out of place" , "Not suitable or appropriate" , "not seemly" .)