Regrets accepting Liberal regulations (5)
I believe the answer is:
rules
'regulations' is the definition.
('rule' can be a synonym of 'regulation')
'regrets accepting liberal' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'accepting' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'rues' going around 'l' is 'RULES'.
(Other definitions for rules that I've seen before include "Australian football" , "governments" , "Reigns" , "Is king" , "game in Australia" .)