Husband's old flame is a witch (3)
I believe the answer is:
hex
'witch' is the definition.
('hex' can be a synonym of 'witch')
'husband's old flame' is the wordplay.
'husband' becomes 'h'.
'old flame' becomes 'ex' (I've seen this in other clues).
'h'+'ex'='HEX'
'is a' acts as a link.
(Other definitions for hex that I've seen before include "A magic spell or curse" , "Bewitch" , "Witch" , "Wicked spell" , "Evil spell" .)