Sleep inside that thing, not suitable (5)
I believe the answer is:
inapt
'not suitable' is the definition.
(I've seen this before)
'sleep inside that thing' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'inside' means one lot of letters goes inside another.
'that thing' becomes 'it'.
'nap' inserted inside 'it' is 'INAPT'.
(Other definitions for inapt that I've seen before include "Not fit, unsuitable" , "Free from deceit" , "badly judged" , "Cartoon duck" , "unqualified" .)