Regret having liberal law (4)
I believe the answer is:
rule
'law' is the definition.
(I know that rule is a type of law)
'regret having liberal' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'having' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'rue' going around 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "Govern, reign over" , "Government; regulation" , "Reign - law" , "Determination" , "Control; printed line" .)