One who regrets taking in Liberal leader (5)
I believe the answer is:
ruler
'leader' is the definition.
(I know that ruler is a type of leader)
'one who regrets taking in liberal' is the wordplay.
'one who regrets' becomes 'ruer' (I've seen this in another clue).
'taking in' is an insertion indicator.
'liberal' becomes 'l' (abbreviation).
'ruer' enclosing 'l' is 'RULER'.
(Other definitions for ruler that I've seen before include "Drawing stick or monarch" , "Thoroughly straight" , "Person in charge of country" , "Monarch or yardstick" , "Straight edge for drawing lines" .)