Missile I had briefly seen in river (7)
I believe the answer is:
trident
'missile' is the definition.
(I've seen this before)
'i had briefly seen in river' is the wordplay.
'i had briefly' becomes 'id' ('I'd' is a contraction of 'I had').
'seen in' means one lot of letters goes inside another.
'river' becomes 'trent' (I've seen this before).
'id' placed into 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "A three-pronged spear as wielded by Neptune" , "Three-pronged weapon" , "Neptune's or Britannia's spear" , "Red tint (anag.)" , "Spear with three prongs" .)