Dandy that's experienced at sea (5)
I believe the answer is:
swell
I believe this clue is a double definition.
'dandy' is the first definition.
(synonyms)
'that's experienced at sea' is the second definition.
(swell can mean a wave)
(Other definitions for swell that I've seen before include "Balloon - movement of the sea's surface" , "Burgeon" , "mushroom" , "Heave of sea; expand" , "Enlarge; excellent" .)