There's a dimmer chance that you'll see one in the theatre! (7)
I believe the answer is:
mermaid
'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" .)