Quick sketch you don't want going under the hammer? (9)
I believe the answer is:
thumbnail
'quick sketch' is the definition.
'thumbnail' can be an answer for 'sketch' (I've seen this before). I'm unsure of the 'quick' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for thumbnail that I've seen before include "small picture" , "Digital process" , "This digital-type sketch is brief and concise" , "Kind of sketch r" , "Sort of sketch" .)