Working railman, true blue (11)
I believe the answer is:
ultramarine
'blue' is the definition.
(I know that ultramarine is a type of blue)
'working railman true' is the wordplay.
'working' is an anagram indicator.
'railman'+'true'='railmantrue'
'railmantrue' with letters rearranged gives 'ULTRAMARINE'.
(Other definitions for ultramarine that I've seen before include "one of the colours" , "Tone" , "Brilliant deep blue" , "Vivid blue" , "Deep blue pigment" .)