Actor's miscast as one of heavenly twins (6)
I believe the answer is:
castor
'one of heavenly twins' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'actor's miscast' is the wordplay.
'miscast' indicates an anagram.
'actors' with letters rearranged gives 'CASTOR'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for castor that I've seen before include "Small swivelled wheel fixed to each leg of a piece of furniture" , "Oil beloved by parents of old" , "Fur of the beaver" , "Brother to Pollux" , "excellent once at the Gabba" .)