Criminal in scarlet pants (9)
I believe the answer is:
larcenist
'criminal' is the definition.
(larcenist is a kind of criminal)
'in scarlet pants' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'in'+'scarlet'='inscarlet'
'inscarlet' is an anagram of 'LARCENIST'.
(Other definitions for larcenist that I've seen before include "Thief" , "One criminal" , "nicker" , "Person who takes" , "Dishonest person" .)