Marker is only beyond motorway, in case (9)
I believe the answer is:
milestone
'marker' is the definition.
(milestone is a kind of marker)
'only beyond motorway in case' is the wordplay.
'only' becomes 'one' ('one' can be a synonym of 'only').
'beyond' is a charade indicator (letters next to each other).
'motorway' becomes 'mi' (resembles M1, English motorway).
'in case' becomes 'lest' (I've seen this before).
'mi'+'lest'='milest'
'one' put after 'milest' is 'MILESTONE'.
'is' acts as a link.
(Other definitions for milestone that I've seen before include "Important stage (reached)" , "Significant stage of development" , "Roadside distance marker" , "Significant developmental stage" , "Wayside indicator" .)