Reverse: regret road crumbling (10)
I believe the answer is:
retrograde
'reverse' is the definition.
(thesaurus)
'regret road crumbling' is the wordplay.
'crumbling' indicates an anagram.
'regret'+'road'='regretroad'
'regretroad' with letters rearranged gives 'RETROGRADE'.
(Other definitions for retrograde that I've seen before include "Moving backwards" , "degenerate" , "Going back" , "Declining" , "Reverse" .)