Man's one in this legion (4)
I believe the answer is:
isle
'man's one' is the definition.
(I've seen this before)
'in this legion' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ISLE' is hidden in the letters of 'this legion'.
(Other definitions for isle that I've seen before include "A poetic island" , "Land amid water" , "eg Man, Ely" , "Atoll" , "Little land" .)