Gum spread round bottom of canvas (5)
I believe the answer is:
paste
'gum' is the definition.
(I've seen this before)
'spread round bottom of canvas' is the wordplay.
'spread' becomes 'pate' (pate is a kind of spread).
'round' means one lot of letters goes inside another.
'bottom of' says to take the final letters.
The final letter of 'canvas' is 's'.
'pate' going around 's' is 'PASTE'.
(Other definitions for paste that I've seen before include "Stick using tapes" , "Adhesive; imitation gem" , "Liquid glue" , "Watery adhesive" , "Imitation gems; adhesive" .)