Move to stop explosive missile (7)
I believe the answer is:
trident
'missile' is the definition.
(UK nuclear weapons system)
'move to stop explosive' is the wordplay.
'move' becomes 'ride' (both can mean to travel).
'to stop' means one lot of letters goes inside another.
'explosive' becomes 'tnt' (TNT is an example).
'ride' inserted into 'tnt' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Neptune's fork" , "Spear held by Neptune or Britannia" , "Submarine missile" , "3-pronged spear" , "A three-pronged spear as wielded by Neptune" .)