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' means one lot of letters go next to another.
'republican' becomes 'R' (abbreviation).
'l' going inside '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" .)