See unfit fruit in the vineyard (3)
I believe the answer is:
cru
'the vineyard' is the definition.
(I've seen this in another clue)
'see unfit fruit' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'unfit fruit' becomes 'ru' (I can't explain this - if you can you should believe this answer much more).
'c'+'ru'='CRU'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cru that I've seen before include "Vintage, as applied to wine" , "Group of vineyards" , "French vineyard or group" , "Superior vineyard" , "Vineyard or group of them in France" .)