Mummy's God transformed Nubia's black dog (6)
I believe the answer is:
anubis
'mummy's god' is the definition.
'Anubis' can be an answer for 'god' (Anubis is an example). I'm not sure about the 'mummy's' bit.
'transformed nubia's black dog' is the wordplay.
'transformed' indicates an anagram.
'black dog' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'nubia' with letters rearranged gives 'anubi'.
'anubi'+'s'='ANUBIS'
Can you help me to learn more?
(Other definitions for anubis that I've seen before include "Egyptian god - in a bus (anag)" , "Egyptian deity" , "Egypt. jackal-headed god" , "Jackal-headed deity" , "Is a bun about the ancient Egyptian god of the dead" .)