Moving in the same direction but always the same distance apart (8)
I believe the answer is:
parallel
'moving in the same direction but always the same distance apart' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for parallel that I've seen before include "Side by side" , "Analogy; line of latitude" , "Of lines, continually at same distance from each other" , "Imaginary line round the Earth" , "Analogue; similarity" .)