Bun seed (6)
I believe the answer is:
sesame
'bun seed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sesame that I've seen before include "Source of seed and oil" , "Open ...... is magic phrase for opening door in pantomime" , "Plant yielding seeds used for flavouring bread" , "Open .... is a pantomime cry" , "Plant with oil-rich seeds" .)