A number flipped over for part of graph (4)
I believe the answer is:
axis
'for part of graph' is the definition.
(as in X or Y axis)
'a number flipped' is the wordplay.
'number' becomes 'six' (six is a kind of whole number).
'flipped' says the letters should be written backwards.
'six' in reverse letter order is 'xis'.
'a'+'xis'='AXIS'
'over' is the link.
(Other definitions for axis that I've seen before include "Imaginary line around which a body rotates" , "Imaginary central line" , "vertebra" , "Imaginary dividing line" , "all revolves around this" .)