Walk back a bit - about a mile (5)
I believe the answer is:
tramp
'walk' is the definition.
(tramp is a kind of walk)
'back a bit about a mile' is the wordplay.
'back' says the letters should be written backwards.
'a bit' becomes 'part' (I've seen this in other clues**).
'about' indicates putting letters inside.
'a mile' becomes 'm' (common abbreviation).
'part' placed around 'm' is 'pmart'.
'pmart' written backwards gives 'TRAMP'.
(Other definitions for tramp that I've seen before include "Beggar or vagrant" , "Down-and-out" , "The man on the street?" , "Vagrant, hobo" , "woman who's promiscuous?" .)