Roadside pillar indicating Northumberland far away? (9)

I believe the answer is:

milestone

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'roadside pillar' is the definition.
'milestone' can be an answer for 'pillar' (I've seen this before). I am unsure of the 'roadside' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for milestone that I've seen before include "Significant developmental stage" , "Significant event or stage - a measure" , "Significant stage of development" , "Important stage (reached)" , "Roadside distance marker" .)

I've seen this clue in The Telegraph.

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