Spy conceals missile in oil-producing plant (6)
I believe the answer is:
sesame
'oil-producing plant' is the definition.
(I've seen this before)
'spy conceals missile' is the wordplay.
'spy' becomes 'see' (spy can mean to watch or see).
'conceals' indicates putting letters inside.
'missile' becomes 'SAM' (abbreviation for surface-to-air missile).
'see' going around 'sam' is 'SESAME'.
'in' is the link.
(Other definitions for sesame that I've seen before include "Seed used in flavouring" , "Password of the Arabian Nights" , "A seed; a TV Street" , "Opening word for a magician" , "Source of seed and oil" .)