Watch out! The only good reason that the examiner would ask your wife, the petitioner, for a police clearance would be if he thinks that she has a criminal record and might be deportable.
Better check this out
There is no such thing as a "police clearance" in adjustment of status cases. Ask your wife if she was arrested - even if the charges were withdrawn, dismissed etc. Go the court where she appeared and ask the criminal court clerk for a disposition of arrest. If she does have a criminal record but did not disclose it in her application or at the interview, you will receive a letter from USCIS. If that happens, see an immigration lawyer immediately. Good luck.