Diehards bring up a drug, oddly (9)
I believe the answer is:
rearguard
'diehards bring up a drug oddly' is the definition.
I can't tell whether this definition defines the answer.
'bring up a drug oddly' is the wordplay.
'bring up' becomes 'rear' (rearing is a kind of bringing up).
'oddly' indicates an anagram.
'a'+'drug'='adrug'
'adrug' with letters rearranged gives 'guard'.
'rear'+'guard'='REARGUARD'
Can you help me to learn more?
(Other definitions for rearguard that I've seen before include "Defensive" , "Troops protecting back of military formation" , "detachment" , "conservative faction" , "conservative element" .)