Mostly growing fruit (6)
I believe the answer is:
raisin
'fruit' is the definition.
(raisin is a kind of fruit)
'mostly growing' is the wordplay.
'mostly' means to remove the last letter.
'growing' becomes 'raising' ('raise' can be a synonym of 'grow').
'raising' with its final letter taken off is 'RAISIN'.
(Other definitions for raisin that I've seen before include "A dried grape, nice in cake" , "Dried grape used in cakes" , "A dried grape used in cake" , "Cultivating" , "Dried red grape" .)