Regularly suing peer is not being fair (7)
I believe the answer is:
unequal
'not being fair' is the definition.
(inequality is a lack of fairness)
'regularly suing peer' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'peer' becomes 'equal' (synonyms).
The alternating letters of 'suing' are 'un'.
'un'+'equal'='UNEQUAL'
'is' is the link.
(Other definitions for unequal that I've seen before include "Dissimilar" , "Poorly balanced or matched" , "Varying" , "Disproportionate" , "Differing" .)