Notice missile entering that plant (6)
I believe the answer is:
sesame
'plant' is the definition.
(sesame is a kind of plant)
'notice missile entering' is the wordplay.
'notice' becomes 'see' (I've seen this before).
'missile' becomes 'sam' (SAM is an example).
'entering' indicates putting letters inside.
'see' placed around 'sam' is 'SESAME'.
'that' is the link.
(Other definitions for sesame that I've seen before include "Plant cultivated for its seeds - opening word?" , "Plant with oily seeds" , "Crop valued for its seeds and oil" , "Ali Baba's opening command" , "Open . . . . . . cries the magician" .)