Opening word? (6)
I believe the answer is:
sesame
'opening word?' 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 "Crop valued for its seeds and oil" , "Source of seed and oil" , "Open ...... is magic phrase for obtaining access" , "Open . . . . . . is a magic phrase to gain entry" , "Seed or oil used in cooking" .)