The box will do to put the painting in (6)
I believe the answer is:
carton
'the box' is the definition.
(carton is a kind of box)
'do to put the painting in' is the wordplay.
'do' becomes 'con' (both can mean to defraud).
'to put the painting' becomes 'art' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'con' going around 'art' is 'CARTON'.
'will' is the link.
Can you help me to learn more?
(Other definitions for carton that I've seen before include "fictional lawyer" , "Cardboard or plastic container" , "Box made of cardboard" , "He did a far, far better thing" , "Container for food or liquid" .)