Regretting taking large decision (6)
I believe the answer is:
ruling
'decision' is the definition.
(thesaurus)
'regretting taking large' is the wordplay.
'regretting' becomes 'ruing' ('rue' can be a synonym of 'regret').
'taking' indicates putting letters inside.
'large' becomes 'l'.
'ruing' going around 'l' is 'RULING'.
(Other definitions for ruling that I've seen before include "Decree" , "judgment" , "currently in force" , "Verdict" , "Drawing a fine line" .)