Spy housing missile in oil-producing plant (6)
I believe the answer is:
sesame
'oil-producing plant' is the definition.
(I've seen this before)
'spy housing missile' is the wordplay.
'spy' becomes 'see' (spy can mean to watch or see).
'housing' is an insertion indicator.
'missile' becomes 'SAM' (abbreviation for surface-to-air missile).
'see' going around 'sam' is 'SESAME'.
'in' acts as a link.
(Other definitions for sesame that I've seen before include "Password of the Arabian Nights" , "Open . . . . . . cries the magician" , "said to be effective in opening spell" , "Oil plant from Ali Baba's cry" , "Seeds used for making oil" .)