Man being one inside Boris leaves (4)
I believe the answer is:
isle
'man' is the definition.
(I've seen this before)
'one inside boris leaves' is the wordplay.
'one inside' says the answer is hidden in the clue.
'ISLE' can be found hidden inside 'boris leaves'.
'being' is the link.
(Other definitions for isle that I've seen before include "Rum, for example" , "Anglesey, say" , "Lies about small land mass." , "Eg, eyot or inch" , "Small landmass" .)