At sea? Isn't the wrong number? (12)
I believe the answer is:
anaesthetist
'number?' is the definition.
(I've seen this before)
'at sea? isn't the wrong' is the wordplay.
'wrong' indicates an anagram.
'at'+'sea'+'isnt'+'the'='atseaisntthe'
'atseaisntthe' anagrammed gives 'ANAESTHETIST'.
(Other definitions for anaesthetist that I've seen before include "Hospital theatre worker" , "One preparing patients for operations" , "Number" .)