Dog, say, to depart — barking (8)
I believe the answer is:
tetrapod
'dog' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?
'to depart barking' is the wordplay.
'barking' is an anagram indicator (UK slang for mad).
'to'+'depart'='todepart'
'todepart' anagrammed gives 'TETRAPOD'.
'say' is the link.
Can you help me to learn more?
(Other definitions for tetrapod that I've seen before include "breakwater" , "Four-footed animal" , "Any animal with four limbs" .)