Drug that's fine being taken in church (4)
I believe the answer is:
coke
'drug' is the definition.
(coke is a kind of drug)
'fine being taken in church' is the wordplay.
'fine' becomes 'ok' (synonyms).
'being taken in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'ok' placed inside 'ce' is 'COKE'.
'that's' acts as a link.
(Other definitions for coke that I've seen before include "Substance left after gas and tar have been extracted from coal" , "Carbonised fuel residue" , "something for heating" , "Drug (abbr) - soft drink (abbr)" , "Solid fuel - drink - drug" .)