The last round — great! (5)
I believe the answer is:
omega
'the last' is the definition.
(thesaurus)
'round great' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'great' becomes 'mega' (I've seen this before).
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "end of series" , "in Greece, the limit!" , "Ultimate letter" , "symbol of resistance?" , "Art of story-telling by body movement" .)