Regret bringing in Latin canon (4)
I believe the answer is:
rule
'canon' is the definition.
(thesaurus)
'regret bringing in latin' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'bringing in' indicates putting letters inside.
'latin' becomes 'l'.
'rue' placed around 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "Regulation, control" , "Govern with straight stick?" , "Determination" , "line to be followed" , "Control, government" .)