Witnesses unaffected by reversal (4)
I believe the answer is:
sees
This clue is a double definition.
'witnesses' is the first definition.
('see' can be a synonym of 'witness')
'unaffected by reversal' is the second definition.
(Other definitions for sees that I've seen before include "Notices" , "Clocks" , "Understands" , "Looks (both ways!)" , "Bishoprics" .)