One who regrets maybe taking in Liberal leader (5)
I believe the answer is:
ruler
'leader' is the definition.
(I've seen this before)
'one who regrets maybe taking in liberal' is the wordplay.
'one who regrets maybe' becomes 'ruer' (someone who rues things).
'taking in' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'ruer' going around 'l' is 'RULER'.
(Other definitions for ruler that I've seen before include "Measuring stick" , "Thoroughly straight" , "Emperor - measure" , "Queen, say" , "Straight edge; monarch" .)