Silly to accept ultimately poor sketch (5)
I believe the answer is:
draft
'sketch' is the definition.
(I know that draft is a type of sketch)
'silly to accept ultimately poor' is the wordplay.
'silly' becomes 'daft' (I've seen this before).
'to accept' means one lot of letters goes inside another.
'ultimately poor' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'daft' placed around 'r' is 'DRAFT'.
Can you help me to learn more?
(Other definitions for draft that I've seen before include "Preliminary text or plan" , "Written plan" , "form of conscription" , "Rough sketch" , "select group" .)