Lose ground as learner crashes into rear end (9)
I believe the answer is:
backslide
'lose ground' is the definition.
(thesaurus)
'learner crashes into rear end' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'crashes into' indicates putting letters inside.
'rear end' becomes 'backside' (synonyms).
'l' going inside 'backside' is 'BACKSLIDE'.
'as' is the link.
(Other definitions for backslide that I've seen before include "Lapse into bad habits" , "Degenerate" , "Relapse into bad ways" .)