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 Ask your wife if she was arrested - even if the charges were withdrawn, dismissed etc.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.
Immigration Green cards Adjustment of immigration status Immigrant status Criminal charges and immigration status Criminal defense Criminal charges Defenses for criminal charges Criminal arrest Criminal court Criminal record Form I-485 (adjustment of status) Spousal immigration Marriage-based visa