Slander REM as "pants" (5)
I believe the answer is:
smear
'slander' is the definition.
('smear' can be a synonym of 'slander')
'rem as pants' is the wordplay.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'rem'+'as'='remas'
'remas' with letters rearranged gives 'SMEAR'.
(Other definitions for smear that I've seen before include "Dirty mark; a libel" , "Slur - may be studied microscopically" , "sample on slide" , "unpleasant mark" , "Blur, smudge" .)