Deck in a thousand-tonner at sea (8)
I believe the answer is:
ornament
'deck' is the definition.
(thesaurus)
'a thousand-tonner at sea' is the wordplay.
'thousand' becomes 'm' ('m' can be a synonym of 'thousand').
'at sea' indicates anagramming the letters.
'a'+'m'+'tonner'='amtonner'
'amtonner' is an anagram of 'ORNAMENT'.
'in' is the link.
(Other definitions for ornament that I've seen before include "Something decorative" , "composers can provide in a shake" , "It's used to adorn" , "grace" , "Turn" .)