Some system users about to restart (6)
I believe the answer is:
resume
'restart' is the definition.
('resume' can be a synonym of 'restart')
'some system users about' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'about' is a reversal indicator.
'emuser' is hidden in the letters of 'system users'.
'emuser' in reverse letter order is 'RESUME'.
'to' is the link.
(Other definitions for resume that I've seen before include "proceed" , "Continue after pause" , "Continue - summary" , "list of achievements in Washington?" , "Start or take up again" .)