One’s run into stubborn guy offering traditional Christmas festivities (7)

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

An envelope (‘into’) of I”S (‘one’s’) plus R (‘run’) in MULE (‘stubborn guy’). In mediaeval times, the Lord of Misrule was appointed to oversee Christmastide revelries, and the New Year Feast of Fools.