Transform one with a pain reliever (7)
I believe the answer is:
morphia
'pain reliever' is the definition.
(old-fashioned term for morphine)
'transform one with a' is the wordplay.
'transform' becomes 'morph' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'morph'+'i'+'a'='MORPHIA'
(Other definitions for morphia that I've seen before include "Opiate" , "Sedative" , "Painkilling drug" , "hypnotic drug" .)