Last character round, playing game (5)
I believe the answer is:
omega
'last character' is the definition.
(Greek alphabet)
'round playing game' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'playing' indicates an anagram.
'game' anagrammed gives 'mega'.
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "Final Greek letter" , "Last thing" , "Last (Gk. letter)" , "in Greece, the limit!" , "16 letter" .)