Observe missile entering plant (6)
I believe the answer is:
sesame
'plant' is the definition.
(sesame is a kind of plant)
'observe missile entering' is the wordplay.
'observe' becomes 'see' (I've seen this before).
'missile' becomes 'sam' (SAM is an example).
'entering' is an insertion indicator.
'see' enclosing 'sam' is 'SESAME'.
(Other definitions for sesame that I've seen before include "Seeds used for making oil" , "Open . . . . . . is a magic phrase to gain entry" , "Plant grown for its seeds" , "'Open . . . . . .'' is a magic phrase to gain access" , "Source of seed and oil" .)