Lad following college concert (6)
I believe the answer is:
unison
'concert' is the definition.
(to work in concert is to work in unison)
'lad following college' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'following' is a charade indicator (letters next to each other).
'college' becomes 'uni' (short for university).
'son' put after 'uni' is 'UNISON'.
(Other definitions for unison that I've seen before include "Concordance" , "As one, singers perhaps" , "interval?" , "Agreement or concord, as with choir" , "Complete agreement, harmony" .)