One's experienced men only in later years coming round (3,6)
I believe the answer is:
old stager
'one's experienced' is the definition.
'men only in later years coming round' is the wordplay.
'men only' becomes 'stag' (I've seen this before).
'in later years' becomes 'older'.
'coming round' means one lot of letters goes inside another.
'stag' inserted inside 'older' is 'OLD STAGER'.
(Other definitions for old stager that I've seen before include "veteran" , "expert" , "senior citizen" , "One who's experienced a lot" .)