Prohibit an American fruit (6)
I believe the answer is:
banana
'fruit' is the definition.
(banana is a kind of fruit)
'prohibit an american' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'ban'+'an'+'a'='BANANA'
(Other definitions for banana that I've seen before include "Small politically unstable country dominated by foreign companies" , "Yellow-skinned fruit" , "Curved fruit (in a split dessert?)" , "Another of 5 down" , "Fruit item" .)