Rule guiding one's conduct in clipper at sea (9)
I believe the answer is:
principle
'rule guiding one's conduct' is the definition.
'principle' can be an answer for 'rule' (I have seen 'General rule' mean 'principle' so perhaps 'rule' could also mean 'principle'). I'm not certain of the 'guiding one's conduct' bit.
'in clipper at sea' is the wordplay.
'at sea' is an anagram indicator.
'in'+'clipper'='inclipper'
'inclipper' with letters rearranged gives 'PRINCIPLE'.
Can you help me to learn more?
(Other definitions for principle that I've seen before include "Morally correct behaviour" , "Set of rules" , "Basic standard" , "Precept, rule" , "Basic truth, law or assumption" .)