Matilda keeps artist in sack (6)
I believe the answer is:
maraud
'sack' is the definition.
(I've seen this before)
'matilda keeps artist' is the wordplay.
'matilda' becomes 'maud' (I can't justify this - if you can you should give a lot more credence to this answer).
'keeps' indicates putting letters inside.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'maud' enclosing 'ra' is 'MARAUD'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for maraud that I've seen before include "Rove to plunder" , "Go searching for people to attack" , "Sack" , "Rove around raiding" , "Raid and plunder" .)