Rough sea: RN comes close (5)
I believe the answer is:
nears
'comes close' is the definition.
(I've seen this in another clue)
'rough sea rn' is the wordplay.
'rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'sea'+'rn'='searn'
'searn' is an anagram of 'NEARS'.
(Other definitions for nears that I've seen before include "is approaching" , "advances" , "Uses snare to get closer" , "Saner (anag.)" , "Moves closer to" .)