Hole flipping full of rats? Don’t worry about it! (5,4)
I believe the answer is:
never mind
'don't worry about it' is the definition.
(equivalent phrases)
'hole flipping full of rats?' is the wordplay.
'hole' becomes 'den' (I've seen this in other clues).
'flipping' says the letters should be written in reverse.
'full of' indicates putting letters inside.
'rats?' becomes 'vermin' (rats are vermin).
'den' back-to-front is 'ned'.
'ned' enclosing 'vermin' is 'NEVER MIND'.
(Other definitions for never mind that I've seen before include "Forget" , "who cares?" , "Don't worry" , "It doesn't matter" .)