Mature split with sweetheart (4)
I believe the answer is:
ripe
'mature' is the definition.
(I know that mature can be written as ripe)
'split with sweetheart' is the wordplay.
'split' becomes 'rip' (synonyms).
'with' is a charade indicator (letters next to each other).
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'rip'+'e'='RIPE'
(Other definitions for ripe that I've seen before include "Fully developed or matured" , "green? No!" , "Ready to harvest" , "Right" , "Mature, ready for eating" .)