There's only a dimmer possibility that she exists! (7)
I believe the answer is:
mermaid
'possibility that she exists' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'there's only a dimmer' is the wordplay.
'a dimmer' anagrams to 'MERMAID'.
Though, I'm unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for mermaid that I've seen before include "Fabulous woman" , "'Half woman, half fish (7)'" , "Imaginary woman of the sea" , "Mythical sea dweller" , "siren" .)