Liberal, in regret, meeting Republican leader (5)
I believe the answer is:
ruler
'leader' is the definition.
(I've seen this before)
'liberal in regret meeting republican' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'in' indicates putting letters inside.
'regret' becomes 'rue' (to rue something is to regret it).
'meeting' is a charade indicator (letters next to each other).
'republican' becomes 'R' (abbreviation in US politics).
'l' put within 'rue' is 'rule'.
'rule'+'r'='RULER'
(Other definitions for ruler that I've seen before include "Aid for a straight line" , "establishing a line?" , "Head of state" , "leader in government" , "Strip with straight edge to measure" .)