Going back in, quickly fill unlimited void (7)
I believe the answer is:
nullify
'void' is the definition.
(I know that void can be written as nullify)
'going back in quickly fill unlimited' is the wordplay.
'going back' says the letters should be written backwards.
'in' indicates a hidden word.
'yfillun' can be found hidden inside 'quickly fill unlimited'.
'yfillun' in reverse letter order is 'NULLIFY'.
(Other definitions for nullify that I've seen before include "Neutralise" , "Make of no effect" , "Render void" , "Declare invalid" , "In fully (anag.)" .)