Source of bun seeds (6)
I believe the answer is:
sesame
'source of bun seeds' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sesame that I've seen before include "'Open . . . . . .'' is a magic phrase to gain access" , "Plant cultivated for its seeds - opening word?" , "Plant with edible seeds" , "Opening word for a magician" , "Open ...... is magic phrase for opening door in pantomime" .)