Student notices youngsters (4)
I believe the answer is:
lads
'youngsters' is the definition.
(I've seen this in another clue)
'student notices' is the wordplay.
'student' becomes 'L' (as in L-plates for learner drivers).
'notices' becomes 'ads' (advertisements).
'l'+'ads'='LADS'
(Other definitions for lads that I've seen before include "Youngsters" , "Fellows, blokes" , "Young men" , "Fellows, chaps" , "Boys, young fellows" .)