Opposing allegation in a court Clem rubbished (12)
I believe the answer is:
counterclaim
'opposing allegation' is the definition.
'in a court clem rubbished' is the wordplay.
'rubbished' indicates anagramming the letters.
'in'+'a'+'court'+'clem'='inacourtclem'
'inacourtclem' anagrammed gives 'COUNTERCLAIM'.
(Other definitions for counterclaim that I've seen before include "Attempt to rebut charge" , "assertive response" , "demand in opposition to another" .)