Regrets holding Liberal principles (5)
I believe the answer is:
rules
'principles' is the definition.
('rule' can be a synonym of 'principle')
'regrets holding liberal' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'holding' 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 "game in Australia" , "Australian football" , "Laws; draws lines" , "In Australia, football" , "instructions" .)