Crossword Genius

There's a dimmer chance that you'll see one in the theatre! (7)

Ross

I believe the answer is:

mermaid

I'm a little stuck... Click here to teach me more about this clue!

'one in the theatre' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'there's a dimmer chance that you'll' is the wordplay.
'a dimmer' can be anagrammed to 'MERMAID'.
However, I am unsure how the anagram is indicated.

'see' acts as a link.

Can you help me to learn more?

(Other definitions for mermaid that I've seen before include "siren" , "Sea-girl!" , "London theatre?" , "Mythical sea creature" , "Fabulous woman" .)

I've seen this clue in the Evening Standard.
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