What is a clo, and how much does it really weigh?

1 tog is 0.1 m2K/W.
It is a measure of thermal insulation per unit area. 2 square meters of clothing with a value of 10 togs will lose heat at a rate of 100 watts when the outside temperature is 50degC below skin temperature.

1 clo = 1.55 togs, which is that of a 'standard british business suit'.

How much does it really need to weigh?

You hear this quoted alot. "10 pound of clothing = 1.5 clo". Rubbish. That is only true if you insist on backpacking in 1.5 standard british business suits.

How about a medium wool sweater and 200wt fleece pants, each with a light nylon shell over it, and maybe a light skin layer underneath? And how about some hats and mitts and thicker socks, and turn in the oxford shoes for trail runner?

Perhaps 2.5 pounds in total, not counting the shell or trail runners.
The shell is neccessary to get the full clo, but I would add that weight extra.

But what's the clo or tog value?

According to this article, see the last page...
http://dspace.ncaor.org:8080/dspace/bitstream/123456789/291/3/article31.pdf

Dead air has a value of 2.9 togs/cm, and clothing insulation achieves a value of about 2.44 togs/cm. That works out to about 1.5 clo per cm thickness, or 4 clo per inch. I would guess that you might get about 1/2" of loft from the above described outfit, of about 2.5 togs. So a good rule of thumb for an adult is a clothing insulation weight of 1 pound per Tog to cover the entire body of a large adult hiker, plus the weight of your shell and boots. This assumes very even coverage including head, hands, and feet, and your insulation is loose and not overly dense or compressed.

1 pounds of full body clothing insulation = 1 tog.
So for 1/2" of clothing loft for 4 togs of winter clothing you would need 4 pounds of wool, fleece, and base layers, plus another 2 pounds for shell layers and footwear. For 8 togs you would need 8 pounds plus 2 pounds for 10 pounds, or perhaps some of that extra 4 togs could be down, which might be 2 togs per pound including its own shell layers. In summer you might only need 1 tog but it might weigh 2 pounds because of thin and uneven coverage, plus you still need another 2 pounds for shell and footwear.

Total clothing weight as a function of minimum temperature?
This is just a guess now...
50F = 1 pounds + shell and footwear = 3 pounds, maybe less but who cares its so little.
30F = 2 pounds + shell and footwear = 4 pounds, but can be very wet also at 30F ???
10F = 3 pounds + shell and footwear = 5 pounds
-10F = 4 pounds + shell and footwear = 6 pounds
-30F = 5 pounds + shell and footwear = 7 pounds