One on relapse with clean drug (5,4)
I believe the answer is:
angel dust
'drug' is the definition.
(angel dust is a kind of drug)
'one on relapse with clean' is the wordplay.
'one on relapse' becomes 'angel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'clean' becomes 'dust' (dusting is a kind of cleaning).
'angel'+'dust'='ANGEL DUST'
Can you help me to learn more?
(Other definitions for angel dust that I've seen before include "Drug" , "Hallucinogen - stale dung (anag)" .)