Never mind vacuum? (2,6)
I believe the answer is:
no matter
I believe this clue is a double definition.
'never mind' is the first definition.
(I've seen this in another clue)
'vacuum?' is the second definition.
(I've seen this before)
(Other definitions for no matter that I've seen before include "What about it, it's unimportant" , "It makes no difference" , "pure energy?" , "Never mind" , "It doesn't make any difference" .)