Not there on the borders of Hertfordshire (4)
I believe the answer is:
here
'not there' is the definition.
('here' is the opposite to 'there')
'the borders of hertfordshire' is the wordplay.
'the borders of' suggests removing the centre.
'hertfordshire' with its centre taken out is 'HERE'.
'on' acts as a link.
(Other definitions for here that I've seen before include ""___ be dragons" (old map inscription?)" , "final clue?" , "On this spot" , "In this place - present" , "At this stage" .)