Embargo on developing a missile (9)
I believe the answer is:
boomerang
'a missile' is the definition.
(boomerang is a kind of missile)
'embargo on developing' is the wordplay.
'developing' indicates an anagram.
'embargo'+'on'='embargoon'
'embargoon' anagrammed gives 'BOOMERANG'.
(Other definitions for boomerang that I've seen before include "Curved throwing stick" , "Australian throwing stick" , "Primitive weapon" , ""It keeps coming back"" , "Self-returning missile" .)