Prohibit an Australian fruit (6)
I believe the answer is:
banana
'fruit' is the definition.
(banana is a kind of fruit)
'prohibit an australian' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'australian' becomes 'a'.
'ban'+'an'+'a'='BANANA'
(Other definitions for banana that I've seen before include "Curved yellow fruit" , "Elongated edible fruit" , "Curved fruit (in a split dessert?)" , "Tropical yellow fruit" , "Plantain-like fruit" .)