Accepting a bit of silver Judas set up King? (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

RAT (Judas = slang for a traitor), containing the first letter (a bit) of S [ilver], reversed (set up = upwards in a down clue). Slavic title for a monarch, equivalent to King, though the derivation is from Latin Caesar = emperor.