Plain Welsh seaside resort, no dud (5)
I believe the answer is:
llano
'plain' is the definition.
(a grassy plain)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for llano that I've seen before include "Treeless area" , "Flat area" , "Treeless grassy plain in South America" , "Grassy plain in South?America" , "(South American) grassy plain" .)