Sesame (3)
'sesame' is the definition.
(I've seen this before)
This is all the clue.
(Another definition for til that I've seen is " Sesame".)
(Other definitions for seed that I've seen before include "Pip; tennis player" , "Top tennis player" , "one very much fancied" , "Plant reproducer" , "Germ; beginning" .)
(Other definitions for open that I've seen before include "gaping" , "William --, English poet" , "Not shut up or hidden" , "Frank and unconcealed" , "Exposed; free" .)