Three-pronged spear, free, found in canvas shelter (7)
I believe the answer is:
trident
'three-pronged spear' is the definition.
(I've seen this in another clue)
'free found in canvas shelter' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'found in' is an insertion indicator.
'canvas shelter' becomes 'tent' (I've seen this before).
'rid' put inside 'tent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Nuclear weapon" , "Three-pronged weapon" , "Neptune's or Britannia's spear" , "A three-pronged spear as wielded by Neptune" , "Submarine missile" .)