It's quite a distance by motorway, boy! (5)
I believe the answer is:
miles
'it's quite a distance' is the definition.
(I've seen this in another clue)
'by motorway boy' is the wordplay.
'by' says to put letters next to each other.
'motorway' becomes 'MI' (resembles Ml, English motorway).
'boy' becomes 'les' (short for Leslie).
'mi' put next to 'les' is 'MILES'.
(Other definitions for miles that I've seen before include "a long way" , "'But I have promises to keep, and . . . . . to go before I sleep' (Frost)" , "Imperial units of length" , "Distance units" .)