A very long time back in the ring (5)
I believe the answer is:
arena
'the ring' is the definition.
(ring is a kind of arena or venue)
'a very long time back' is the wordplay.
'a' becomes 'an'.
'very long time' becomes 'era' (I've seen this before).
'back' shows that the letters should be reversed in order.
'an'+'era'='anera'
'anera' written backwards gives 'ARENA'.
'in' acts as a link.
(Other definitions for arena that I've seen before include "Olympic venue?" , "Area surrounded by seating, for sports, etc." , "bowl? Possibly" , "Earls Court?" , "Sporting enclosure" .)