Canon's in clipper at sea (9)
I believe the answer is:
principle
'canon's' is the definition.
(thesaurus)
'in clipper at sea' is the wordplay.
'at sea' indicates an anagram.
'in'+'clipper'='inclipper'
'inclipper' is an anagram of 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Precept, rule" , "honour" , "Set of rules" , "General guiding rule" , "Morally correct behaviour" .)