On reflection regretted taking to diversion (6)
I believe the answer is:
detour
'diversion' is the definition.
(thesaurus)
'on reflection regretted taking to' is the wordplay.
'on reflection' is a reversal indicator.
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'taking' indicates putting letters inside.
'rued' backwards is 'deur'.
'deur' enclosing 'to' is 'DETOUR'.
(Other definitions for detour that I've seen before include "Go round an obstruction" , "Roundabout route, to avoid something say" , "Routed (anag.)" , "Indirect route" , "Diversion" .)