Silly frame for Rembrandt's capital sketch (5)
I believe the answer is:
draft
'sketch' is the definition.
(I know that draft is a type of sketch)
'silly frame for rembrandt's capital' is the wordplay.
'silly' becomes 'daft' (I've seen this in another clue).
'frame for' indicates putting letters inside (outside letters frame or surround others).
'capital' suggests taking the first letters.
The initial letter of 'rembrandt' is 'r'.
'daft' placed around 'r' is 'DRAFT'.
(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" .)