Green promises after beginnings of exciting new vows (7)
I believe the answer is:
envious
'green' is the definition.
(green with envy)
'promises after beginnings of exciting new vows' is the wordplay.
'promises' becomes 'IOUs' (promises to pay a debt).
'after' says to put letters next to each other.
'beginnings of' indicates taking the first letters (the beginning letters).
The first letters of 'exciting new vows' is 'env'.
'ious' put after 'env' is 'ENVIOUS'.
(Other definitions for envious that I've seen before include "Green-eyed" , "Mildly jealous" , "Feeling jealousy" , "Full of jealousy" , "Begrudging another's fortune" .)