Nasty smell clears up before church (6)
I believe the answer is:
stench
'nasty smell' is the definition.
(I've seen this before)
'clears up before church' is the wordplay.
'clears' becomes 'nets' (as in to make a net profit).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'before' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
'nets' in reverse letter order is 'sten'.
'sten'+'ch'='STENCH'
(Other definitions for stench that I've seen before include "Nasty smell" , "Reek" , "Strong unpleasant smell" , "Unpleasant odour" , "Stink" .)