Regret bringing in Liberal government (4)
I believe the answer is:
rule
'government' is the definition.
(I've seen this before)
'regret bringing in liberal' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'bringing in' indicates putting letters inside.
'liberal' becomes 'l' (abbreviation).
'rue' enclosing 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "line to be followed" , "Govern through lure" , "Determination" , "Reign - law" , "Government; regulation" .)