Clumsy model sat for Rembrandt's first great painting (3,6)
I believe the answer is:
old master
'great painting' is the definition.
'old master' can be an answer for 'painting' (I've seen this before). I'm unsure of the 'great' bit.
'clumsy model sat for rembrandt's first' is the wordplay.
'clumsy' indicates an anagram.
'for' is a charade indicator (letters next to each other).
'rembrandt's first' becomes 'r' (1st letter of 'rembrandt').
'model'+'sat'='modelsat'
'modelsat' with letters rearranged gives 'oldmaste'.
'oldmaste'+'r'='OLD MASTER'
Can you help me to learn more?
(Other definitions for old master that I've seen before include "Great painter of former times - or one of his works" , "Costa?" , "Classic painting" , "Do armlets serve for great painter of yore?" , "Could get a picture" .)