Responsible youth’s pressure upset lout (4,3)
I believe the answer is:
head boy
'responsible youth's' is the definition.
(school pupil with special responsibilities)
'pressure upset lout' is the wordplay.
'pressure' becomes 'head' (head is a kind of pressure).
'upset' shows that the letters should be reversed in order.
'lout' becomes 'yob' (both can mean a rude or criminal young person).
'yob' reversed gives 'boy'.
'head'+'boy'='HEAD BOY'
(Other definitions for head boy that I've seen before include "Top pupil" , "Most advanced pupil" .)