Mark’s seldom seen missing church (4)
I believe the answer is:
scar
'mark's' is the definition.
(mark left by an injury)
'seldom seen missing church' is the wordplay.
'seldom seen' becomes 'scarce' (I've seen this in another clue).
'missing' indicates named letters should be taken away.
'church' becomes 'CE' (abbreviation for Church of England).
'scarce' with 'ce' taken out is 'SCAR'.
(Other definitions for scar that I've seen before include "fall could possibly be the reason for one" , "Mark, blemish" , "Sign of wound" , "Evidence of a past injury?" , "Mark left by injury" .)