One is foul, wicked criminal (9)
I believe the answer is:
felonious
'criminal' is the definition.
(I know that criminal can be written as felonious)
'one is foul wicked' is the wordplay.
'wicked' indicates anagramming the letters.
'one'+'is'+'foul'='oneisfoul'
'oneisfoul' with letters rearranged gives 'FELONIOUS'.
(Other definitions for felonious that I've seen before include "Criminal - foul noise (anag)" , "Involved in crime" , "Involving serious crime - foul noise (anag)" .)