Folk art regularly is dull (4)
I believe the answer is:
flat
'dull' is the definition.
(synonyms)
'folk art regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'folk'+'art'='folkart'
The alternating letters of 'folkart' are 'FLAT'.
'is' acts as a link.
(Other definitions for flat that I've seen before include "Digs perhaps" , "Uninteresting - piece of scenery" , "In music, below the correct pitch" , "character on the staff" , "Not sloping; apartment" .)