Helmsman in endless river redirected missile (6)
I believe the answer is:
exocet
'missile' is the definition.
(I know that Exocet is a type of missile)
'helmsman in endless river redirected' is the wordplay.
'helmsman' becomes 'cox' (coxswain on a boat).
'in' indicates putting letters inside.
'endless river' becomes 'tee' (I can't justify this - if you can you should believe this answer much more).
'redirected' says the letters should be written in reverse (going in the other direction).
'cox' going within 'tee' is 'tecoxe'.
'tecoxe' back-to-front is 'EXOCET'.
Can you help me to learn more?
(Other definitions for exocet that I've seen before include "One was guided" , "Tactical missile" , "Subsonic, low-altitude missile" , "Short range guided missile" , "French anti-ship guided missile" .)