Regret stocking large line (4)
I believe the answer is:
rule
'line' is the definition.
(thesaurus)
'regret stocking large' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'stocking' indicates putting letters inside ('stock' can mean 'carry' or 'contain').
'large' becomes 'l'.
'rue' placed around 'l' is 'RULE'.
(Other definitions for rule that I've seen before include "Government; regulation" , "Strip" , "Regulation, custom" , "Govern with straight stick?" , "line to be followed" .)