Thirteen artists with lead in their pencils? (7)
I believe the answer is:
drawers
'artists with lead in their pencils?' is the definition.
(people who draw with pencils)
I cannot understand how the rest of the clue works.
(Other definitions for drawers that I've seen before include "Underwear (for artists?)" , "designers" , "Undergarments" , "Old-fashioned clothes" , "Underpants or sketchers" .)