An opening word (6)
I believe the answer is:
sesame
'an 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 "'Open - - !' (Ali Baba)" , "Plant with oily seeds" , "Seeds producing an edible oil" , "Open . . . . . . is a magic phrase to gain entry" , "Open ...... is magic phrase for obtaining access" .)