Provide food initially to everyone boarding railway carriage (5)
I believe the answer is:
cater
'provide food' is the definition.
(I've seen this before)
'initially to everyone boarding railway carriage' is the wordplay.
'initially' says to take the initial letters.
'boarding' is an insertion indicator.
'railway carriage' becomes 'car' (car can mean a train carriage).
The first letters of 'to everyone' is 'te'.
'te' going into 'car' is 'CATER'.
(Other definitions for cater that I've seen before include "Provide services, food or entertainment" , "Supply provisions" , "Supply food for parties" , "Provide food etc" , "Trace in order to serve" .)