Sleep during sex - that's not appropriate (5)
I believe the answer is:
inapt
'not appropriate' is the definition.
(inapt means inappropriate or ill-fitting)
'sleep during sex' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'during' indicates putting letters inside.
'sex' becomes 'it' (a common euphemism).
'nap' inserted within 'it' is 'INAPT'.
'that's' is the link.
(Other definitions for inapt that I've seen before include "Ill-suited" , "Improper" , "Not suitable or appropriate" , "Unsuitable - paint (anag)" , "not seemly" .)