Painter's canvas finally framed by old measure of length (6)
I believe the answer is:
cubist
'painter's' is the definition.
(cubist is a kind of painter)
'canvas finally framed by old measure of length' is the wordplay.
'finally' indicates one should take the final letters.
'framed by' indicates putting letters inside.
'old measure of length' becomes 'cubit' (I've seen this before).
The last letter of 'canvas' is 's'.
's' placed into 'cubit' is 'CUBIST'.
(Other definitions for cubist that I've seen before include "Artist as Picasso, Braque" , "Picasso, e.g." , "modern artist" , "Early twentieth century style painter" , "Artist like Picasso" .)