Old murderer found in church gets drug (7)
I believe the answer is:
cocaine
'drug' is the definition.
(cocaine is a kind of drug)
'old murderer found in church' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'murderer' becomes 'cain' (Cain who killed Abel).
'found in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'o'+'cain'='ocain'
'ocain' going into 'ce' is 'COCAINE'.
'gets' is the link.
(Other definitions for cocaine that I've seen before include "Addictive drug of oceanic origin" , "Strange oceanic narcotic" , "Number" , "A narcotic, anaesthetic" , "Addictive narcotic" .)