Silly frame for Rembrandt's capital sketch (5)
I believe the answer is:
draft
'capital sketch' is the definition.
'draft' can be an answer for 'sketch' (draft is a kind of sketch). I am unsure of the 'capital' bit.
'silly frame for rembrandt's' is the wordplay.
'silly' becomes 'daft' (I've seen this in another clue).
'frame for' means one lot of letters goes inside another (inserted letters are framed by others).
'rembrandt' becomes 'r' (this might be a standard abbreviation which I don't know about).
'daft' enclosing 'r' is 'DRAFT'.
Can you help me to learn more?
(Other definitions for draft that I've seen before include "Sketch; money order" , "Draw an outline" , "Formulate" , "Current of air or preliminary sketch" , "Preliminary drawing" .)