A cupful of fruit (5)
I believe the answer is:
acorn
'fruit' is the definition.
(I know that acorn is a type of nut)
'a cupful' is the wordplay.
'cupful' becomes 'corn' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'corn'='ACORN'
'of' is the link.
Can you help me to learn more?
(Other definitions for acorn that I've seen before include "Might source" , "old computer?" , "Potential oak" , "Model in stone" , "From which a tall tree may grow" .)