Witness corrupt cop at rest (9)
I believe the answer is:
spectator
'witness' is the definition.
(I know that witness can be written as spectator)
'corrupt cop at rest' is the wordplay.
'corrupt' indicates an anagram (a corrupted spelling of the word).
'cop'+'at'+'rest'='copatrest'
'copatrest' anagrammed gives 'SPECTATOR'.
(Other definitions for spectator that I've seen before include "given weekly inspection?" , "Onlooker at game" , "member of the crowd" , "Witness" , "One watching a game" .)