Promise not to be unfaithful again? (7)
I believe the answer is:
remarry
'promise' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'not to be unfaithful again?' is the wordplay.
I cannot quite understand how this works, but
'again?' could be 're' (prefix meaning 'again') and 're' is present in the answer.
The remaining letters 'marry' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for remarry that I've seen is " Wed again".)