Revolutionary study protects pests? It doesn't matter (5,4)
I believe the answer is:
never mind
'it doesn't matter' is the definition.
(I've seen this before)
'revolutionary study protects pests?' is the wordplay.
'revolutionary' says the letters should be written backwards (letters revolve around).
'study' becomes 'den' (den can mean an office or study).
'protects' indicates putting letters inside (outer letters protect inside letters).
'pests?' becomes 'vermin' (I've seen this before).
'den' written backwards gives '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" .)