Drawing implement (6)
(Other definitions for pencil that I've seen before include "implement in case?" , "Implement for writing, drawing etc" , "Writing instrument" , "American bureaucrat -- purple niches (anag)" , "HB writer?" .)
'drawing implement' is the definition.
(crayons are used for drawing)
This is all the clue.
(Other definitions for crayon that I've seen before include "One colours with this" , "Writer" , "Producer of sketch" , "Pencil of chalk or wax" , "pencil (13) [FRENCH]" .)