A sailor, a philosopher, and a dog (4,7)
I believe the answer is:
jack russell
'a dog' is the definition.
(I've seen this before)
'a sailor a philosopher' is the wordplay.
'a sailor' becomes 'jack' (informal term for a sailor).
'a philosopher' becomes 'russell' (Bertrand Russell).
'jack'+'russell'='JACK RUSSELL'
'and' is the link.
(Other definitions for jack russell that I've seen before include "dog" , "Small working terrier" , "Breed of terrier" , "Former England wicketkeeper" .)