Embargo on developing primitive weapon (9)
I believe the answer is:
boomerang
'primitive weapon' is the definition.
(I've seen this before)
'embargo on developing' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'embargo'+'on'='embargoon'
'embargoon' anagrammed gives 'BOOMERANG'.
(Other definitions for boomerang that I've seen before include "Self-returning missile" , "Aboriginal projectile" , "Curved throwing stick" , ""It keeps coming back"" , "Recoil from" .)