Scene of combat a long time back (5)
I believe the answer is:
arena
'scene of combat' is the definition.
(I've seen this before)
'a long time back' is the wordplay.
'a' becomes 'an'.
'long time' becomes 'era' (a long period of time).
'back' says the letters should be written backwards.
'an'+'era'='anera'
'anera' back-to-front is 'ARENA'.
(Other definitions for arena that I've seen before include "battlefieId" , "Battleground" , "Place of great activity" , "What the Colosseum was" , "Earls Court?" .)