Join cetacean (not at sea) (11)
I believe the answer is:
concatenate
'join' is the definition.
(thesaurus)
'cetacean not at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'cetacean'+'not'='cetaceannot'
'cetaceannot' with letters rearranged gives 'CONCATENATE'.
(Other definitions for concatenate that I've seen before include "String together" , "Link together in a chain" .)