Learner breaks smoothing tool? Calumny! (7)
I believe the answer is:
slander
'calumny' is the definition.
(slander is a kind of calumny)
'learner breaks smoothing tool?' is the wordplay.
'learner' becomes 'l'.
'breaks' is an insertion indicator.
'smoothing tool?' becomes 'sander' (I've seen this before).
'l' put inside 'sander' is 'SLANDER'.
(Other definitions for slander that I've seen before include "muckraking!" , "Calumny" , "False or malicious report" , "False malicious spoken statement" , "Defame" .)