Pineapple is fruit, but not British (6)
I believe the answer is:
ananas
'pineapple' is the definition.
(synonyms)
'is fruit but not british' is the wordplay.
'is fruit' becomes 'bananas' (I've seen this before).
'but not' is a deletion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'bananas' with 'b' taken out is 'ANANAS'.
(Other definitions for ananas that I've seen before include "Sort of fruit" , "Black top on this fruit" , "Pineapple" , "piece of fruit" , "F" .)