Train regularly plain blue (6)
I believe the answer is:
ribald
'blue' is the definition.
(synonyms)
'train regularly plain' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'plain' becomes 'bald' (I've seen this before).
The alternate letters of 'train' are 'ri'.
'ri'+'bald'='RIBALD'
(Other definitions for ribald that I've seen before include "Scurrilous, bawdy" , "Coarse" , "Coarsely humorous; scurrilous" , "Mean" , "Licentious" .)