Defend vessel in contest (12)
I believe the answer is:
championship
'contest' is the definition.
(thesaurus)
'defend vessel' is the wordplay.
'defend' becomes 'champion' (synonyms).
'vessel' becomes 'ship' (ship is a kind of vessel).
'champion'+'ship'='CHAMPIONSHIP'
'in' is the link.
(Other definitions for championship that I've seen before include "Sporting contest to decide overall winner" , "title" , "Contest - advocacy" .)