Rock is first thing for teenagers after school (6)
I believe the answer is:
schist
'rock' is the definition.
(I know that schist is a type of rock)
'is first thing for teenagers after school' is the wordplay.
'first thing for' indicates taking the first letters.
'after' says to put letters next to each other.
'school' becomes 'sch' (abbreviation).
The first letter of 'teenagers' is 't'.
'is'+'t'='ist'
'ist' after 'sch' is 'SCHIST'.
(Other definitions for schist that I've seen before include "Form of rock" , "layered rock" , "Coarse-grained rock" .)