Popular music, something often encountered at the seaside (4)
I believe the answer is:
rock
I believe this clue is a double definition.
'popular music' is the first definition.
(genre of music)
'something often encountered at the seaside' is the second definition.
(rock is a sweet sold at the seaside)
(Other definitions for rock that I've seen before include "Stagger" , "Edinburgh - -" , "Be unstable; lull (baby)" , "Cork (anag.)" , "Brighton - (novel); - Hudson" .)