Outline drawing (6)
'outline drawing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sketch that I've seen before include "An outline drawing or a comedy number" , "result of quick draw?" , "Basic drawing" , "Short humorous scene" , "Small sailing boat" .)
(Other definitions for silhouette that I've seen before include "Outline figure" , "Shadowy outline" , "Outline representation" , "Profile of characters" , "image" .)