Nick heavy weight up on church (5)
I believe the answer is:
notch
'nick' is the definition.
('notch' can be a synonym of 'nick')
'heavy weight up on church' is the wordplay.
'heavy weight' becomes 'ton' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
'ton' back-to-front is 'not'.
'not'+'ch'='NOTCH'
(Other definitions for notch that I've seen before include "Indentation, nick" , "Incision on an edge" , "V-shaped incision" , "Salad plant, garnish" , "Secure" .)