Trifle prepared with British nut (7)
I believe the answer is:
filbert
'nut' is the definition.
(I know that filbert is a type of edible nut)
'trifle prepared with british' is the wordplay.
'prepared with' is an anagram indicator.
'british' becomes 'b' (abbreviation e.g. in BBC).
'trifle' anagrammed with 'b' gives 'FILBERT'.
(Other definitions for filbert that I've seen before include "Nut -- left rib (anag)" , "Hazel nut" , "Type of hazelnut" .)