A rare drug ordered for defenders at the back (9)
I believe the answer is:
rearguard
'defenders at the back' is the definition.
'a rare drug ordered' is the wordplay.
'ordered' indicates anagramming the letters.
'a'+'rare'+'drug'='araredrug'
'araredrug' is an anagram of 'REARGUARD'.
'for' is the link.
(Other definitions for rearguard that I've seen before include "detachment" , "Defensive" , "Troops protecting back of military formation" , "Plaza-Toro's place" , "conservative element" .)