Mark departs European school, upset (6)
I believe the answer is:
denote
'mark' is the definition.
(thesaurus)
'departs european school upset' is the wordplay.
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'european' becomes 'e' (abbreviation e.g. EU).
'school' becomes 'eton' (famous UK public school**).
'upset' says the letters should be written in reverse.
'eton' backwards is 'note'.
'd'+'e'+'note'='DENOTE'
(Other definitions for denote that I've seen before include "Need to (anag) - indicate" , "Be the sign or symbol of" , "Mean or signify" , "Designate" , "Stand as symbol for" .)