A rude song at sea — dicey! (9)
I believe the answer is:
dangerous
'dicey' is the definition.
(similar in meaning)
'a rude song at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'a'+'rude'+'song'='arudesong'
'arudesong' anagrammed gives 'DANGEROUS'.
(Other definitions for dangerous that I've seen before include "Unsafe, risky" , "Posing a hazard" , "Perilous, unsafe" , "menacing" , "Pope: 'A little learning is a . . . . . . . . . thing'" .)