Weapon, free, found in portable shelter (7)
I believe the answer is:
trident
'weapon' is the definition.
(trident is a kind of weapon)
'free found in portable shelter' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'found in' means one lot of letters goes inside another.
'portable shelter' becomes 'tent' (I've seen this before).
'rid' going inside 'tent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Red tint (anag.)" , "Neptune's fork" , "Neptune's or Britannia's spear" , "Fork perhaps" , "A three-pronged spear as wielded by Neptune" .)