Whiff of something unpleasant in impetuous senior pupil (4,3)
I believe the answer is:
head boy
'senior pupil' is the definition.
(senior male student)
'whiff of something unpleasant in impetuous' is the wordplay.
'whiff of something unpleasant' becomes 'BO' (body odour).
'in' is an insertion indicator.
'impetuous' becomes 'heady' (I've seen this before).
'bo' inserted within 'heady' is 'HEAD BOY'.
(Other definitions for head boy that I've seen before include "Most advanced pupil" , "Top pupil" .)