Competitive place a long time back (5)
I believe the answer is:
arena
'competitive place' is the definition.
(where people compete)
'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 in reverse.
'an'+'era'='anera'
'anera' in reverse letter order is 'ARENA'.
(Other definitions for arena that I've seen before include "Venue for boxing, circus and other action events" , "place for sporting contest" , "Where gladiatorial contests took place" , "Area of combat" , "sports ground" .)