Fashionable cosmetic preparation for one vocalising in church (7)
I believe the answer is:
intoner
'one vocalising in church' is the definition.
The definition suggests a singular noun which matches the answer.
'fashionable cosmetic preparation' is the wordplay.
'fashionable' becomes 'in' (similar in meaning).
'cosmetic preparation' becomes 'toner' (I can't explain this - if you can you should give a lot more credence to this answer).
'in'+'toner'='INTONER'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for intoner that I've seen before include "Participant in service" , "eg, a monk?" , "Someone who speaks monotonously - iron net (anag)" , "chanter" .)