Regret route regularly (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'route regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'route' are 'RUE'.
(Other definitions for rue that I've seen before include "Regret in a French street" , "Repentant sort of shrub?" , "Swann's way?" , "Bitter-tasting herb" , "Lament; a herb" .)