Man perhaps? One's left sweetheart (4)
I believe the answer is:
isle
'man perhaps?' is the definition.
(I've seen this before)
'one's left sweetheart' is the wordplay.
'one's' becomes 'is' (roman numeral).
'left' becomes 'l' (common abbreviation).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'is'+'l'+'e'='ISLE'
(Other definitions for isle that I've seen before include "Sea-girt area" , "'There is an ..., a bonny ...' (rugby anthem in the South)" , "you can't walk away from" , "Sea-girt land" , "eg Man, Wight" .)