Not appropriate to sleep inside it (5)
I believe the answer is:
inapt
'not appropriate' is the definition.
(inapt means inappropriate or ill-fitting)
'sleep inside it' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'inside' is an insertion indicator.
'nap' going within 'it' is 'INAPT'.
'to' acts as a link.
(Other definitions for inapt that I've seen before include "Ill-chosen, not suitable" , "Not fitting" , "Far from fit" , "Useless" , "Inappropriate" .)