Rebecca's back, with regret about grand feud (5)
I believe the answer is:
argue
'feud' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'rebecca's back with regret about grand' is the wordplay.
'back' suggests the final letters (the letter at the back).
'with' is a charade indicator (letters next to each other).
'regret' becomes 'rue' (to rue something is to regret it).
'about' is an insertion indicator.
'grand' becomes 'g' (G is short for grand as in a thousand).
The final letter of 'rebecca' is 'a'.
'rue' enclosing 'g' is 'rgue'.
'a'+'rgue'='ARGUE'
Can you help me to learn more?
(Other definitions for argue that I've seen before include "Dispute heatedly" , "express disagreement" , "Bandy words" , "Fall out" , "Altercate" .)