Regularly roast scoundrel (3)
I believe the answer is:
rat
'scoundrel' is the definition.
(rat can mean a nasty person)
'regularly roast' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'roast' are 'RAT'.
(Other definitions for rat that I've seen before include "Abandon" , "Rodent; traitor" , "Strike-breaker" , "Betrayer" , "Domestic rodent" .)