Incredible woman of the theatre (7)
I believe the answer is:
mermaid
'the theatre' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'incredible woman' is the wordplay.
I cannot really understand how this works, but
'woman' could be 'maid' (maid is a kind of woman) and 'maid' is present in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for mermaid that I've seen before include "'Half woman, half fish (7)'" , "Sea-girl!" , "siren" , "Fabulous female" , "Fabulous woman" .)