Numbers crazy about fruit (6)
I believe the answer is:
damson
'fruit' is the definition.
(damson is a kind of fruit)
'numbers crazy about' is the wordplay.
'numbers' becomes 'nos' (no. is a kind of number**).
'crazy' becomes 'mad' ('mad' can be a synonym of 'crazy').
'about' is a reversal indicator.
'nos'+'mad'='nosmad'
'nosmad' backwards is 'DAMSON'.
(Other definitions for damson that I've seen before include "jam producer" , "might one be in a jam?" , "Wild plum" , "Ma, Don's a fruit" , "Small dark-purple plum" .)