Weapon that's free, wrapped in canvas (7)
I believe the answer is:
trident
'weapon' is the definition.
(trident is a kind of weapon)
'free wrapped in canvas' is the wordplay.
'free' becomes 'rid' ('rid' can be a synonym of 'free').
'wrapped in' means one lot of letters goes inside another.
'canvas' becomes 'tent' (I've seen this in other clues).
'rid' placed within 'tent' is 'TRIDENT'.
'that's' is the link.
(Other definitions for trident that I've seen before include "Britannia's weapon" , "Spear held by Neptune or Britannia" , "Red tint? (anagram)" , "Neptune's fork" , "Pronged spear" .)