Man being one within his league (4)
I believe the answer is:
isle
'man' is the definition.
(I've seen this before)
'one within his league' is the wordplay.
'one within' indicates the answer is hidden within the clue.
'ISLE' is hidden in the letters of 'his league'.
'being' acts as a link.
(Other definitions for isle that I've seen before include "Man conceivably" , "Wight, Man or Skye" , "Small landmass" , "Man, maybe" , "what Donne implied he wasn't" .)