A lunatic capsized sailing (8)
I believe the answer is:
nautical
'sailing' is the definition.
(thesaurus)
'a lunatic capsized' is the wordplay.
'capsized' is an anagram indicator.
'a'+'lunatic'='alunatic'
'alunatic' with letters rearranged gives 'NAUTICAL'.
(Other definitions for nautical that I've seen before include "Like 8 21[HORATIO NELSON]" , "Seafaring" , "A lunatic (anag)" , "about boats" , "Maritime" .)