Unsuitable to sleep in it (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(thesaurus)
'sleep in it' is the wordplay.
'sleep' becomes 'nap' (I've seen this before).
'in' indicates putting letters inside.
'nap' inserted within 'it' is 'INAPT'.
'to' is the link.
(Other definitions for inapt that I've seen before include "Free from deceit" , "Cartoon duck" , "Ill-chosen, not suitable" , "Not suitable or appropriate" , "Not proper" .)