Mother's boy's a fruit (6)
I believe the answer is:
damson
'a fruit' is the definition.
(damson is a kind of fruit)
'mother's boy's' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'dam'+'son'='DAMSON'
(Other definitions for damson that I've seen before include "fruit tree" , "Stone fruit for nomads" , "Acid purple plum" , "Type of plum" , "sort of fruit" .)