At sea, certain ensign displays stubbornness (13)
I believe the answer is:
intransigence
'stubbornness' is the definition.
(intransigence is a kind of stubbornness)
'at sea certain ensign' is the wordplay.
'at sea' is an anagram indicator.
'certain'+'ensign'='certainensign'
'certainensign' anagrammed gives 'INTRANSIGENCE'.
'displays' is the link.
(Other definitions for intransigence that I've seen before include "pigheadedness" , "Stubbornness" , "lack of flexibility" , "Unwillingness to agree" , "Stubborn refusal to compromise" .)