Girl probably losing heart (4)
I believe the answer is:
lily
'girl' is the definition.
(girl's name)
'probably losing heart' is the wordplay.
'probably' becomes 'likely' (synonyms).
'losing heart' means to remove the middle letters.
'likely' with its middle taken out is 'LILY'.
(Other definitions for lily that I've seen before include "Easter flower" , "Bulbous plant with trumpet-shaped flowers" , "Fleur-de-lis" , "Flower; girl's name" , "'Behold the . . . . of the field'" .)