Error suppressed by City Police (4)
I believe the answer is:
typo
'error' is the definition.
(typographical error)
'suppressed by city police' is the wordplay.
'suppressed by' indicates the answer is hidden within the clue.
'TYPO' is hidden within 'city police'.
(Other definitions for typo that I've seen before include "Character flaw" , "Printing error (colloq.)" , "Misprint" , "Small printing error" , "Mistake in printed matter, in short" .)