Act like a student after bad mark horror (5)
I believe the answer is:
dread
'horror' is the definition.
(thesaurus)
'act like a student after bad mark' is the wordplay.
'act like a student' becomes 'read'.
'after' means one lot of letters go next to another.
'bad mark' becomes 'd'.
'read' put after 'd' is 'DREAD'.
(Other definitions for dread that I've seen before include "Be scared of" , "Affright" , "great anxiety" , "Trepidation" , "Dared to fear greatly" .)