Some say terrible judge gives mark for changing letters (9)
I believe the answer is:
diacritic
'mark for changing letters' is the definition.
(a diacritic is a mark which changes a letter)
'some say terrible judge' is the wordplay.
'some say' indicates a 'sounds like' (homophone) clue.
'terrible' becomes 'dire' (synonyms).
'judge' becomes 'critic' (critic is a kind of judge).
'dire' sounds like 'dia'.
'dia'+'critic'='DIACRITIC'
'gives' acts as a link.
(Other definitions for diacritic that I've seen before include "Capable of distinguishing" , "[ACCENT] , perhaps" , "Mark" , "accent, for example" , "grave perhaps" .)