One’s under canvas to relax at length (5)
I believe the answer is:
easel
'one's under canvas' is the definition.
(an easel might hold up a canvas)
'relax at length' is the wordplay.
'relax' becomes 'ease' (I've seen this before).
'at' is a charade indicator (letters next to each other).
'length' becomes 'l'.
'ease'+'l'='EASEL'
'to' acts as a link.
(Other definitions for easel that I've seen before include "the rest of the paintwork" , "Lease out the stand" , "Constable's stand?" , "artist's requisite" , "Artist's wooden frame" .)