Where milestones may be seen in passing (2,3,3)
I believe the answer is:
by the way
'in passing' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for by the way that I've seen before include "At the side of the path?" , "In passing, while we're on the subject" , "Where to find kerbstones" , "In point of fact" , "At the roadside - incidentally" .)