What does the cryptic crossword clue "It's hard, running along the motorway! (8)" mean?

The definition is "It's hard, running along the motorway!". The wordplay uses a cryptic definition. Cryptic definition The answer is SHOULDER.

It's hard, running along the motorway! (8)

Guardian Cryptic #29267 | 21 Across | 2024-01-01 flower