Firstly selecting carriage, hop onto Circle Line train (6)
I believe the answer is:
school
'train' is the definition.
(both can mean to teach or coach)
'firstly selecting carriage hop onto circle line' is the wordplay.
'firstly' indicates taking the first letters.
'onto' means one lot of letters go next to another.
'circle line' becomes 'ool' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'selecting carriage hop' is 'sch'.
'sch'+'ool'='SCHOOL'
Can you help me to learn more?
(Other definitions for school that I've seen before include "Teach - fishes swimming" , "Large group of fish" , "Train - a group of fish" , "classes?" , "Educational institution" .)