In a journey, part of the eye that helps night vision (10)
I believe the answer is:
flashlight
'that helps night vision' is the definition.
(a flashlight helps you see at night)
'in a journey part of the eye' is the wordplay.
'in' means one lot of letters goes inside another.
'a journey' becomes 'flight' (flight is a kind of journey).
'part of the eye' becomes 'lash' (eyelash).
'flight' placed around 'lash' is 'FLASHLIGHT'.
(Other definitions for flashlight that I've seen before include "shiner" , "One beams brightly" , "It may be needed to see" , "Electric torch" , "Beamer" .)