Reactionaries behind a drug bust (9)
I believe the answer is:
rearguard
'reactionaries' is the definition.
(rearguard can mean a group of reactionaries)
'behind a drug bust' is the wordplay.
'behind' becomes 'rear' (both can mean one's bottom).
'bust' is an anagram indicator.
'a'+'drug'='adrug'
'adrug' is an anagram of 'guard'.
'rear'+'guard'='REARGUARD'
(Other definitions for rearguard that I've seen before include "conservative element" , "Troops protecting back of military formation" , "conservative faction" , "detachment" , "Defensive" .)