Attendant always accommodating queen on railway (7)
I believe the answer is:
equerry
'attendant' is the definition.
(equerry is a kind of attendant)
'always accommodating queen on railway' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'accommodating' means one lot of letters goes inside another.
'queen' becomes 'qu' (**).
'on' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'eer' placed around 'qu' is 'equer'.
'equer'+'ry'='EQUERRY'
(Other definitions for equerry that I've seen before include "Personal attendant of a member of the royal family" , "Royal family official" , "Royal aide" , "Attendant in royal household" , "Sovereign's officer" .)