Seed the opening word! (6)
I believe the answer is:
sesame
I believe this is a double definition.
'seed' is the first definition.
(sesame seed is a kind of seed)
'opening word' is the second definition.
(I've seen this in another clue)
'the' acts as a link.
(Other definitions for sesame that I've seen before include "Plant with oil-rich seeds" , "Small oval seeds yielding an edible oil" , "Open . . . . . . is a magic phrase to gain entry" , "Plant grown for its seeds" , "'Open - - !' (Ali Baba)" .)