Mark day institute gets a judge (9)
I believe the answer is:
diacritic
'mark' is the definition.
(diacritic is a kind of mark)
'day institute gets a judge' is the wordplay.
'day' becomes 'd'.
'institute' becomes 'i' (common abbreviation as in MIT).
'gets' is a charade indicator (letters next to each other).
'judge' becomes 'critic' (critic is a kind of judge**).
'd'+'i'+'a'+'critic'='DIACRITIC'
(Other definitions for diacritic that I've seen before include "grave perhaps" , "Mark" , "Capable of distinguishing" , "accent, for example" , "one showing a pronounced difference" .)