Pet bird bachelor's sat on (6)
I believe the answer is:
beagle
'pet' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'bird bachelor's sat on' is the wordplay.
'bird' becomes 'eagle' (eagle is a type of bird).
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'sat on' says to put letters next to each other (in a down clue, some letters go on top of others).
'eagle' after 'b' is 'BEAGLE'.
Can you help me to learn more?
(Other definitions for beagle that I've seen before include "hunting animal with smooth coat" , "Small short-legged hound" , "Small, sturdy breed of hound" , "Hunting dog followed on foot" , "Breed of dog in a glebe" .)