A drug curing pain, sir! (7)
I believe the answer is:
aspirin
'a drug' is the definition.
(aspirin is a kind of drug)
'curing pain sir' is the wordplay.
'curing' indicates anagramming the letters.
'pain'+'sir'='painsir'
'painsir' is an anagram of 'ASPIRIN'.
(Other definitions for aspirin that I've seen before include "Commonly used pain-killer" , "number" , "Common pain tablet" , "Mild pain-killing drug" , "Anti-inflammatory drug" .)