Promising band's gig at arena (10,4)
I believe the answer is:
engagement ring
'promising band's' is the definition.
(I've seen this before)
'gig at arena' is the wordplay.
'gig' becomes 'engagement' (I've seen this in another clue).
'at' means one lot of letters go next to another.
'arena' becomes 'ring' (I've seen this in another clue).
'engagement'+'ring'='ENGAGEMENT RING'
(Other definitions for engagement ring that I've seen before include "Promise of future contract" , "Sign union's promised" , "Proof of commitment" , "Union pledge" , "shows commitment to union" .)