These London companies providing current to railway (6)
I believe the answer is:
livery
'these london companies' is the definition.
(livery companies based in the City of London)
'current to railway' is the wordplay.
'current' becomes 'live'.
'to railway' becomes 'ry' (abbreviation for railway).
'live'+'ry'='LIVERY'
'providing' is the link.
(Other definitions for livery that I've seen before include "Uniform, eg of guild" , "Horse maintenance" , "colours" , "Irritable" , "Uniform - commercial care of horses" .)