Brusque hothead in class (5)
I believe the answer is:
short
'brusque' is the definition.
(I know that brusque can be written as short)
'hothead in class' is the wordplay.
'hothead' becomes 'h' (head letter of 'hot').
'in' is an insertion indicator.
'class' becomes 'sort' (synonyms).
'h' put within 'sort' is 'SHORT'.
(Other definitions for short that I've seen before include "Cause to blow a fuse" , "Of small length" , "Uncivil, brief film" , "Not long enough" , "Literary genre such as those by ..." .)