Luscious currants primarily filling bun with sweetness (5)
I believe the answer is:
dolce
'with sweetness' is the definition.
('dolce' means 'sweet' in Italian)
'luscious currants primarily filling bun' is the wordplay.
'primarily' suggests taking the first letters.
'filling' is an insertion indicator.
'bun' becomes 'doe' (doe can mean a female rabbit or bunny).
The first letters of 'luscious currants' is 'lc'.
'lc' placed inside 'doe' is 'DOLCE'.
(Other definitions for dolce that I've seen before include "To be played sweetly" , "sweetly and softly" , "La ... Vita (Fellini film)" , "Stop" , "Musical instruction to play gently and sweetly" .)