Illegal formation

I just reviewed the replay of Clark's touchdown. He was right tackle and there was no end on the play. The refs are obviously as asleep as the ticats.

He was an eligible receiver, so wasn’t he the end? There must have been 3 ineligible guys on the other side of the center.

You're correct.
There were 3 linemen to the left of the centre, plus a receiver split a couple of yards out. Then 1 lineman to the right of centre, plus Clark.
The formation was legal.
Well disguised because you don't expect the guy 2 over from the centre to be an eligible receiver.

This has already been covered, but strictly speaking the rule is that you have to have at least 7 players on the line of scrimmage, and whoever is at either end is eligible as a receiver (as long as they are wearing an eligible number or have been "declared eligible" through the officials).