Fall behind, walking route (5)
I believe the answer is:
trail
This might be a double definition.
'fall behind' is the first definition.
(trailing is a kind of falling behind)
'walking route' is the second definition.
'trail' can be an answer for 'route' (a trail is a route or path). I am not sure about the 'walking' bit.
Can you help me to learn more?
(Other definitions for trail that I've seen before include "Way" , "Mark left where something has passed - follow it" , "Drag along the ground" , "Drag behind" , "Rough track blazed through wild or hilly country" .)