Biggest canvas is an enormity to put in the post (8)
I believe the answer is:
mainsail
'biggest canvas' is the definition.
'mainsail' can be an answer for 'canvas' (mainsail is a kind of canvas). I'm unsure of the 'biggest' bit.
'an enormity to put in the post' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'enormity' becomes 'nsa' (I am not sure about this - if you are sure you should believe this answer much more).
'to put in' indicates putting letters inside.
'the post' becomes 'mail' ('mail' can be a synonym of 'post').
'i'+'nsa'='insa'
'insa' inserted inside 'mail' is 'MAINSAIL'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for mainsail that I've seen before include "The principal sheet on board" , "Principal ship's canvas" , "key equipment on board" , "Ship's largest canvas" , "Central piece of rigging" .)