A rare drug distributed as a means of defence (9)
I believe the answer is:
rearguard
'a means of defence' is the definition.
(soldiers defending an army)
'a rare drug distributed' is the wordplay.
'distributed' is an anagram indicator.
'a'+'rare'+'drug'='araredrug'
'araredrug' with letters rearranged gives 'REARGUARD'.
'as' is the link.
(Other definitions for rearguard that I've seen before include "Plaza-Toro's place" , "conservative faction" , "Defensive" , "Troops protecting back of military formation" , "detachment" .)