Fellow pupil taking tea on train (10)
I believe the answer is:
schoolmate
'fellow pupil' is the definition.
(schoolmates are pupils at school together)
'tea on train' is the wordplay.
'tea' becomes 'mate' (South American hot drink).
'on' is a charade indicator (letters next to each other).
'train' becomes 'school' (I've seen this before).
'mate' put after 'school' is 'SCHOOLMATE'.
'taking' acts as a link.
(Other definitions for schoolmate that I've seen before include "fishwife?" , "Friends Reunited contact" , "Childhood friend" , "Fellow pupil" , "Young friend" .)