Vows could be as hot! (5)
I believe the answer is:
oaths
'vows' is the definition.
('oath' can be a synonym of 'vow')
'could be as hot' is the wordplay.
'could be' indicates an anagram.
'as'+'hot'='ashot'
'ashot' with letters rearranged gives 'OATHS'.
(Other definitions for oaths that I've seen before include "truths may be established by them" , "Swear words" , "Curses - words of honour?" , "Solemn sworn promises" , "Cursing" .)