Memorial: knock one up in church (8)
I believe the answer is:
cenotaph
'memorial' is the definition.
(cenotaph is a kind of memorial)
'knock one up in church' is the wordplay.
'knock' becomes 'pat' (pat can mean to knock or tap).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates putting letters inside.
'church' becomes 'ch' (common abbreviation).
'pat'+'one'='patone'
'patone' in reverse letter order is 'enotap'.
'enotap' going into 'ch' is 'CENOTAPH'.
(Other definitions for cenotaph that I've seen before include "Monument honouring the fallen" , "War memorial" , "Monument to people whose remains are interred elsewhere" , "construction in Whitehall" , "Monument honouring persons buried elsewhere" .)