Regularly rail, plain coarse (6)
I believe the answer is:
ribald
'coarse' is the definition.
(I've seen this before)
'regularly rail plain' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'plain' becomes 'bald' (I've seen this before).
The alternating letters of 'rail' are 'ri'.
'ri'+'bald'='RIBALD'
(Other definitions for ribald that I've seen before include "Coarsely humorous; scurrilous" , "Foul-mouthed" , "Funny, but not nice" , "Bawdy and vulgar" , "Humorous in vulgar or disrespectful way" .)