Charge 100 in a faulty cause (6)
I believe the answer is:
accuse
'charge' is the definition.
(synonyms)
'100 in a faulty cause' is the wordplay.
'100' becomes 'c' (Roman numeral).
'in' is an insertion indicator.
'a faulty' indicates an anagram.
'cause' is an anagram of 'acuse'.
'c' put into 'acuse' is 'ACCUSE'.
(Other definitions for accuse that I've seen before include "Impeach" , "Charge with a crime" , "Apportion blame" , "Point the finger at" , "Indict, blame wrongdoing on" .)