Nicer tuna at sea? Not sure (9)
I believe the answer is:
uncertain
'not sure' is the definition.
('uncertain' is the opposite to 'sure')
'nicer tuna at sea?' is the wordplay.
'at sea?' indicates an anagram (I've seen 'at sea' mean this).
'nicer'+'tuna'='nicertuna'
'nicertuna' with letters rearranged gives 'UNCERTAIN'.
(Other definitions for uncertain that I've seen before include "Lacking confidence" , "Not completely confident or sure of" , "In doubt" , "Not definitely known" , "Not at all sure or decided" .)