Possible Clues:

• Light weights in UNESCO
• Prevention units?
• Sixteen of these to a lb
• Doesn't take much to weigh down old relatives left out
• Pound it in bits of such
• The little bits in the recipe can affect one's weight
• Once they could make light of us
• At once this gets us the light bits
• Units for measuring mail weights
• Once with us there are ends for 21 across