The most common and most likely answer to this crossword clue is the 7 letter word FOOTPAD
I've seen this crossword clue in The Daily Telegraph
F
O
O
T
P
A
D
More clues leading to the result FOOTPAD
Buddy explains!

The straight part
We know that "Highwayman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case highwayman = footpad.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FOOTPAD mean?
FOOTPAD noun- The soft underside of an animal's paw.
- A medicated bandage for the treatment of corns and warts.
- A thief on foot who robs travellers on the road.
- An unmade, minor walking trail formed only by foot traffic.
- To rob travellers on the road.
- To sneak on foot.