Fruit supplying vitamin, I believe? Not entirely (6)
I believe the answer is:
citrus
'fruit' is the definition.
(citrus is a kind of fruit)
'supplying vitamin i believe? not entirely' is the wordplay.
'supplying vitamin' becomes 'c' (I can't explain this - if you can you should give a lot more credence to this answer).
'believe?' becomes 'trust' (trusting is a kind of believing).
'not entirely' means to remove the last letter (not all of the word).
'trust' with its final letter removed is 'trus'.
'c'+'i'+'trus'='CITRUS'
Can you help me to learn more?
(Other definitions for citrus that I've seen before include "Eg, a lime tree" , "Fruit tree genus" , "Orange tree genus" , "source of fruit" , "sort of fruit tree" .)