Dog biting American man's backside for sport (6)

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

MUTT (dog) containing [biting] A (American) + {ma}N [’s backside]. I had no idea about the definition here but have since found this in SOED: sport (biology): A plant (or part of a plant), animal, etc., which exhibits some abnormal or striking variation from the parent stock or type, esp. in form or colour; a spontaneous mutation; a new variety produced in this way. NHO this.