Two promises and an oath (9)
I believe the answer is:
swearword
'an oath' is the definition.
(I know that oath can be written as swearword)
'two promises' is the wordplay.
'two promises' can mean 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'='SWEARWORD'
'and' is the link.
(Other definitions for swearword that I've seen before include "Oath" , "blast perhaps" , "profanity" .)