Got me curious too - it means wool which has been cleaned
Notes on Yield and Conditioned Weight for Wool
Certain duties in Chapter 51 are based on the “clean kg.” which is the clean yield in kilograms. The HTSUS defines “clean yield,” except for purposes of carbonized fibers, as the absolute clean content (i.e., all that portion of the merchandise which consists exclusively of wool or hair free of all vegetable and other foreign material, containing by weight 12 percent of moisture and 1.5 percent of material removable from the wool or hair by extraction with alcohol, and having an ash content of not over 0.5 percent by weight), less an allowance, equal by weight to 0.5 percent of the absolute clean content plus 60 percent of the vegetable matter present, but not exceeding 15 percent by weight of the absolute clean content, for wool or hair that would ordinarily be lost during commercial cleaning operations. For purposes of carbonized fibers, the term “clean yield” means in the condition as entered.
Wool is often bought on the basis of “,” which is the dry weight plus a fixed percentage of expected moisture content by percent of weight, or “moisture regain.” This is usually 15% to 18% depending on the system employed. A term such as “conditioned at 17%” for a wool purchase would indicate that the fiber was bought on the basis of a moisture regain of 17%. Under CIE 1235/57 (which, although written under the previous valuation system and before the HTSUS, still is instructive), valuation was based on the price per conditioned kilogram, while any specific duty rate was assessed against the actual landed net weight in kilograms, adjusted for clean yield.