Up to a point, it's negative (3)
I believe the answer is:
not
'negative' is the definition.
(I've seen this in another clue)
'up to a point' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a point' becomes 'n' (North).
'to'+'n'='ton'
'ton' back-to-front is 'NOT'.
'it's' is the link.
(Other definitions for not that I've seen before include "It negates a word" , "logically, operation needed" , "Type of logic gate" , "Ton (anag.)" , "In no way, never" .)