City have negligible backing? Not entirely! (6)
I believe the answer is:
geneva
'city' is the definition.
(Geneva is an example)
'have negligible backing? not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'backing?' says the letters should be written in reverse (I've seen 'backing' mean this).
'aveneg' can be found hidden inside 'have negligible'.
'aveneg' back-to-front is 'GENEVA'.
(Other definitions for geneva that I've seen before include "Average in Swiss city" , "Avenge (anag) - type of gin" , "Swiss city, lake" , "Swiss city - Dutch spirit" , "Swiss city housing the Red Cross headquarters" .)