Mark makes attorney sick inside church first (7)
I believe the answer is:
cedilla
'mark' is the definition.
(I know that cedilla is a type of diacritic)
'attorney sick inside church first' is the wordplay.
'attorney' becomes 'DA' (abbreviation for District Attorney).
'sick' becomes 'ill' (similar in meaning).
'inside' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'first' says to put letters next to each other.
'da' enclosing 'ill' is 'dilla'.
'dilla' after 'ce' is 'CEDILLA'.
'makes' is the link.
(Other definitions for cedilla that I've seen before include "for French character" , "Mark suggesting a 'c' should be pronounced an 's'" , "Low mark in French" , "Mark indicating a softened consonant" , "Diacritical mark" .)