Failing to sidetrack learner leaving (6)
I believe the answer is:
defect
'failing' is the definition.
(thesaurus)
'sidetrack learner leaving' is the wordplay.
'sidetrack' becomes 'deflect'.
'learner' becomes 'l'.
'leaving' suggests deleting specific letters.
'deflect' with 'l' taken away is 'DEFECT'.
'to' is the link.
(Other definitions for defect that I've seen before include "Failing - desert" , "Join opponents in a cause" , "Abandon one's country in favour of an opposing one" , "Desert - flaw" , "error" .)