As it happens, railway uniform (6)
I believe the answer is:
livery
'uniform' is the definition.
(livery is a type of formal uniform)
'as it happens railway' is the wordplay.
'as it happens' becomes 'live' (as in live broadcasting).
'railway' becomes 'ry' (abbreviation).
'live'+'ry'='LIVERY'
(Other definitions for livery that I've seen before include "Distinctive uniform once worn by servants of posh people" , "old servants" , "outfit in uniform" , "Uniform, eg of guild" , "The stabling and care of horses" .)