Time for a celebration by a third at sea? (8)
I believe the answer is:
birthday
'time for a celebration' is the definition.
'by a third at sea?' is the wordplay.
'at sea?' indicates an anagram (I've seen 'at sea' mean this).
'by'+'a'+'third'='byathird'
'byathird' with letters rearranged gives 'BIRTHDAY'.
(Other definitions for birthday that I've seen before include "When presents are expected" , "Nothing to wear?" , "when one was delivered" , "When one receives cards" , "Invisible attire?" .)