Arms raised at MMA ring (4)
I believe the answer is:
ammo
'arms' is the definition.
(ammo is a kind of arms)
'raised at mma ring' is the wordplay.
'raised' shows that the letters should be reversed in order (going up in a down clue).
'at' says to put letters next to each other.
'ring' becomes 'O' (letter resembling a ring).
'mma' after 'o' is 'omma'.
'omma' written backwards gives 'AMMO'.
(Other definitions for ammo that I've seen before include "Contents of magazine" , "Bullets, shells, rockets etc" , "Bullets, shells (colloq.)" , "Bullets etc.(informal)" , "round" .)