Fruit by the pound if you want one (6)
I believe the answer is:
litchi
'fruit' is the definition.
(litchi is a kind of fruit)
'by the pound if you want one' is the wordplay.
'by' is a charade indicator (letters next to each other).
'the pound' becomes 'l'.
'if you want' becomes 'itch' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'l'+'itch' is 'litch'.
'litch'+'i'='LITCHI'
Can you help me to learn more?
(Other definitions for litchi that I've seen before include "Chinese tree; its fruit" , "Edible Chinese fruit" , "Asian tree" , "piece of fruit" .)