Promised in marriage, but could be hotter in bed! (9)
I believe the answer is:
betrothed
'promised in marriage' is the definition.
(I've seen this before)
'could be hotter in bed' is the wordplay.
'could be' indicates anagramming the letters.
'in' is an insertion indicator.
'hotter' anagrammed gives 'etroth'.
'etroth' put into 'bed' is 'BETROTHED'.
'but' is the link.
(Other definitions for betrothed that I've seen before include "intended" , "Spoken for" , "Engaged to be married" , "Engaged - fiance" , "Promised in marriage" .)