Improper to take forty winks in it (5)
I believe the answer is:
inapt
'improper' is the definition.
(thesaurus)
'forty winks in it' is the wordplay.
'forty winks' becomes 'nap' ('nap' can be a synonym of 'forty winks').
'in' indicates putting letters inside.
'nap' inserted inside 'it' is 'INAPT'.
'to take' acts as a link.
(Other definitions for inapt that I've seen before include "Not fit, unsuitable" , "not fit for purpose" , "Improper" , "Out of place" , "Unsuited" .)