Where gladiators fought a long time back (5)
I believe the answer is:
arena
'where gladiators fought' 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' backwards is 'ARENA'.
(Other definitions for arena that I've seen before include "Action space in centre of amphitheatre" , "Engagement ring" , "Place of combat" , "Amphitheatre's central space" , "battlefieId" .)