For the actors, or cast, "Sister Ada" is a disaster (8)
I believe the answer is:
anagrams
'disaster' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'for the actors or cast sister ada is a' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for anagrams that I've seen before include "Words with letters transposed to form other words" , "Words turned into other words" , "Words made by rearranging the letters of others" , "Words formed from letters of other words" , ""Sword" from "words" and "seraph" from "phrase", for example" .)