A student wearing earring upset grown-ups (6)
I believe the answer is:
adults
'grown-ups' is the definition.
(I've seen this before)
'a student wearing earring upset' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'wearing' is an insertion indicator.
'earring' becomes 'stud' (I've seen this before).
'upset' shows that the letters should be reversed in order.
'l' going into 'stud' is 'stlud'.
'stlud' back-to-front is 'dults'.
'a'+'dults'='ADULTS'
(Other definitions for adults that I've seen before include "Marure people" , "Mature or fully-grown people" , "Fully developed animals" , "Mature animals" , "Mature beings" .)