Gets a tan at sea, being idle (8)
I believe the answer is:
stagnate
'idle' is the definition.
('stagnate' can be a synonym of 'idle')
'gets a tan at sea' is the wordplay.
'at sea' is an anagram indicator.
'gets'+'a'+'tan'='getsatan'
'getsatan' is an anagram of 'STAGNATE'.
'being' is the link.
(Other definitions for stagnate that I've seen before include "Cease to flow or move" , "Cease activity" , "Show little activity" , "(Of water) become motionless" , "Cease motion" .)