Mark's short memo on church (5)
I believe the answer is:
notch
'mark's' is the definition.
(a notch is a mark or scratch on something)
'short memo on church' is the wordplay.
'short' means to remove the last letter.
'memo' becomes 'note' (I've seen this in another clue).
'on' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'note' with its last letter removed is 'not'.
'not'+'ch'='NOTCH'
(Other definitions for notch that I've seen before include "Salad plant, garnish" , "Indentation - step up" , "Small V-shaped cut or indentation" , "Tallying incision" , "Point or degree in a scale" .)