Band showing promise in battle arena (10,4)
I believe the answer is:
engagement ring
'arena' is the definition.
(I've seen this before)
'band showing promise in battle' is the wordplay.
I cannot quite see how this works, but
'band' could be 'ring' (a ring is a band of metal) and 'ring' is found in the answer.
'promise' could be 'engagement' (engagement is a kind of promise) and 'engagement' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for engagement ring that I've seen before include "arena" , "Promise of future contract" , "What would-be union member might get" , "A sign of commitment" , "Proof of commitment" .)