Atlas, perhaps, setter sent back (3)
I believe the answer is:
god
'atlas perhaps' is the definition.
(atlas is a kind of god)
'setter sent back' is the wordplay.
'setter' becomes 'dog' (a setter is a type of dog).
'sent back' says the letters should be written backwards.
'dog' back-to-front is 'GOD'.
(Other definitions for god that I've seen before include "Worshipped figure" , "Deity" , "Superhuman being" , "Mars or Mercury, say" , "Eg, Quetzalcoatl" .)