Female never seen at a London theatre (7)

I believe the answer is:

mermaid

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'female' is the definition.
(I know that mermaid is a type of female)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mermaid that I've seen before include "Bird with a fish's tail" , "Fabulous woman" , "Mythical sea creature" , "London theatre?" , "Woman with tail of fish" .)

I've seen this clue in the Evening Standard.

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