This follows the route when the army is on the move (5)
I believe the answer is:
march
'the move' is the definition.
(march is a kind of move)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for march that I've seen before include "Spring" , "The month for military movement" , "Month, piece of music" , "Military walking style" , "Month - advance in step" .)