God of the underworld and formerly a planet (5)
I believe the answer is:
pluto
This could be a double definition.
'god of the underworld' is the first definition.
(I've seen this before)
'planet' is the second definition.
(Pluto is an example)
'and formerly a' is the link.
I am not very happy about this link. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for pluto that I've seen before include "Roman god" , "Hollywood creation" , "Dis" , "Sun's remote orbiter" , "Greek god - cartoon dog" .)