Regret accepting Liberal convention (4)
I believe the answer is:
rule
'convention' is the definition.
('rule' can be a synonym of 'convention')
'regret accepting liberal' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'accepting' is an insertion indicator.
'liberal' becomes 'l' (abbreviation).
'rue' enclosing 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "Government; regulation" , "Regulation, control" , "Govern the straight edge?" , "Exercise authority over" , "Dominate" .)