About to reverse country's precedent (4,3)
I believe the answer is:
case law
'precedent' is the definition.
(legal precedent)
'about to reverse country's' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to reverse' says the letters should be written backwards.
'country' becomes 'wales' (country of the UK).
'wales' back-to-front is 'selaw'.
'ca'+'selaw'='CASE-LAW'
(Other definitions for case law that I've seen before include "A lot of precedents" , "It is based on precedent" , "legal precedents" , "Legal precedent" , "It depends on precedents" .)