Still gives Guinevere a bit of backing (4)
I believe the answer is:
even
'still' is the definition.
('even' can be a synonym of 'still')
'gives guinevere a bit of backing' is the wordplay.
'a bit of' indicates a hidden word (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'backing' is a reversal indicator.
'neve' can be found hidden inside 'gives guinevere'.
'neve' in reverse letter order is 'EVEN'.
(Other definitions for even that I've seen before include "That's not odd" , "Pantomime" , "Still - flat" , "Flat - yet" , "without a rise?" .)