Rearing mostly fruit (6)
I believe the answer is:
raisin
'fruit' is the definition.
(raisin is a kind of fruit)
'rearing mostly' is the wordplay.
'rearing' becomes 'raising' ('raise' can be a synonym of 'rear').
'mostly' means to remove the last letter.
'raising' with its final letter taken off is 'RAISIN'.
(Other definitions for raisin that I've seen before include "fruit product" , "Fruit for the Christmas cake" , "A dried grape, nice in cake" , "Partially dried grape" , "An iris (anag)" .)