Seems a low bit of fun, but it's really high (4)
I believe the answer is:
moon
'really high' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'seems a low bit of fun but' is the wordplay.
I cannot really understand how this works, but
'low' could be 'moo' (both can mean to make the noise of a cow) and 'moo' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for moon that I've seen before include "low point" , "Happy to be over this" , "who headed a religious 29 [CULT]" , "down one's pants" , "Reveal rear" .)