Flag hung -- sail at sea (8)
I believe the answer is:
languish
'flag' is the definition.
(thesaurus)
'hung sail at sea' is the wordplay.
'at sea' indicates an anagram.
'hung'+'sail'='hungsail'
'hungsail' anagrammed gives 'LANGUISH'.
(Other definitions for languish that I've seen before include "Long" , "go into decline" , "Droop" , "Pine, waste away" , "Grow weak or feeble" .)