Scrub & rinse clementines, place in water in a large soup pot (NOT aluminum).
Squeeze lemon - reserve juice. Bundle the lemon seeds with cheesecloth and tie it off. Add bundle of seeds and the halved lemon to pot.
Cover the pot and simmer for 2-3 hours until fruit is soft.
Remove pot from heat, cool overnight; discard bundle of lemon seeds, do not strain liquid.
Scoop fruit out of the liquid into a large bowl. Halve the clementines, scoop pulp and seeds (if any) into a strainer set over the pot that was used to boil the fruit. Save all peels. Rub pulp through strainer.
Add sugar and lemon juice to pot. Heat gently, stirring until sugar dissolves, then boil for a few minutes.
Meanwhile, finely slice clementine peel then stir into boiling sugar-lemon syrup. Continue boiling until marmalade reaches setting point (220*F at sea level) then remove pot from heat, skim foam, and let pot sit for 10-15 minutes to distribute peel evenly.
Fill hot, sterilized jars; seal with two-part lids; process in boiling-water bath for 15 minutes (sea level).
Label jars; store in cool, dark place.
SUGGESTION: Use small decorative jars and make Gift Baskets (along with other kitchen preserves or mixes).