Sort of seed that might follow Open for a spell (6)
I believe the answer is:
sesame
I believe this clue is a double definition.
'sort of seed' is the first definition.
'that might follow open for a spell' is the second definition.
(as in 'Open Sesame', spell to open a door)
(Other definitions for sesame that I've seen before include "Plant cultivated for its seeds - opening word?" , "Plant with edible seeds" , "Crop valued for its seeds and oil" , "Plant yielding seeds used for flavouring bread" , "Seed used in flavouring" .)