The one arm row is one of the finest mid- to upper-back exercises. Secondarily, it works your biceps and forearms and, to some degree, your abs and lower back because you must stay tight to row properly.
Watch the video a couple of times (daughter Jennifer demonstrates) before trying it. Then take a solid stance, resting one hand on a sturdy support. With your opposite arm extended, pull the dumbbell or kettlebell toward your hip. Lift and descend smoothly. Stay in control. Exhale as you pull. Inhale as you extend.
No hunching over. Keep a nice normally flat back.
If you are super-setting opposing muscle groups, rowing fits perfectly by alternating your rowing sets with chest press exercises.