A cupful of fruit (5)

I believe the answer is:

acorn

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.

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