Originally Posted by Onyxhorn

/targetenemy used to have the same functionality as tabbing & I don't remember seeing that it was broken by any patches. The following should work for you:

/targetenemy
/cast Cleave

However, I'd personally recommend using an addon like Tidy Plates instead that puts your threat % for each mob in their nameplate and then click the plates to change to the lowest threat target after each cleave instead. It's not automated but you'll ultimately have much better control over all the mobs in the group rather than relying on randomness.