A long time going round the Colosseum? (5)
I believe the answer is:
arena
'the colosseum?' is the definition.
(thesaurus)
'a long time going round' is the wordplay.
'a' becomes 'an'.
'long time' becomes 'era' (I've seen this before).
'going round' is a reversal indicator.
'an'+'era'='anera'
'anera' in reverse letter order is 'ARENA'.
(Other definitions for arena that I've seen before include "Gladiatorial ring" , "place for sporting contest" , "Area surrounded by seating, for sports, etc." , "battlefieId" , "Performance venue" .)