Two promises becoming a curse (5,4)
I believe the answer is:
swear word
'a curse' is the definition.
(I've seen this before)
'two promises' is the wordplay.
'two promises' means two substitutions for 'promise'.
'promise' becomes 'swear' (as in swearing something is true).
'promise' becomes 'word' (as in 'I give you my word').
'swear'+'word'='SWEAR-WORD'
'becoming' is the link.
(Other definitions for swear word that I've seen before include "could be bloody?" , "oath" , "Curse" .)