Locate and speak to Tolfdir . He mentions of some mage causing trouble. He tells you to go deal with him. He points the location on your map, so fast travel there, or to the closest location and go to the marker. Clear out the cave system until you locate the mage .
He’s wearing the College’s uniform, so you’ll identify him quick, as well as he has the marker over him. Kill him and return to Tolfdir. He thanks you for taking care of him and… that’s it. Yeah, no reward, besides what you looted.