A lunatic at sea on a ship? (8)
I believe the answer is:
nautical
'on a ship?' is the definition.
'a lunatic at sea' is the wordplay.
'at sea' indicates an anagram.
'a'+'lunatic'='alunatic'
'alunatic' with letters rearranged gives 'NAUTICAL'.
(Other definitions for nautical that I've seen before include "Of ships and navigation" , "Relating to sailors" , "Concerning sailors" , "Of shipping and navigation" , "A lunatic about sailors or seamanship" .)