Nest turned up full of rats? None of your business (5,4)
I believe the answer is:
never mind
'none of your business' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'nest turned up full of rats?' is the wordplay.
'nest' becomes 'den' (both can mean a lair).
'turned up' says the letters should be written in reverse.
'full of' means one lot of letters goes inside another.
'rats?' becomes 'vermin' (I've seen this before).
'den' reversed gives 'ned'.
'ned' enclosing 'vermin' is 'NEVER MIND'.
Can you help me to learn more?
(Other definitions for never mind that I've seen before include "who cares?" , "It doesn't matter" , "Don't worry" , "Forget" .)