Backslide (6)
'backslide' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for relapse that I've seen before include "Fall back into illness" , "falling back" , "Medical setback" , "A return to illness" , "Deterioration after period of improvement" .)
(Other definitions for regress that I've seen before include "Go backwards, disimprove" , "Return or revert" , "Get worse or go backwards" , "Go back to a lower level" , "Backslide" .)
'backslide' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for revert that I've seen before include "Go back (to topic)" , "Go back to type" , "Go back to former state or habit" , "Backslide, return to former state" , "Go back to a former practice" .)