Not fit to sleep in it (5)
I believe the answer is:
inapt
'not fit' is the definition.
(I've seen this before)
'sleep in it' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'in' indicates putting letters inside.
'nap' inserted inside 'it' is 'INAPT'.
'to' is the link.
(Other definitions for inapt that I've seen before include "Cartoon duck" , "Not fit, unsuitable" , "Inappropriate" , "badly judged" , "Not fitting" .)