Distance indicator in case somebody takes motorway first (9)
I believe the answer is:
milestone
'distance indicator' is the definition.
'milestone' can be an answer for 'indicator' (I have seen 'Wayside indicator' mean 'milestone' so perhaps 'indicator' could also mean 'milestone'). I am not sure about the 'distance' bit.
'in case somebody takes motorway first' is the wordplay.
'in case' becomes 'lest' (eg 'lest we forget').
'somebody' becomes 'one' (one can mean some person).
'takes motorway' becomes 'mi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'first' is a charade indicator (letters next to each other).
'lest'+'one'='lestone'
'lestone' after 'mi' is 'MILESTONE'.
Can you help me to learn more?
(Other definitions for milestone that I've seen before include "Significant event or stage - a measure" , "Wayside indicator" , "Pillar" , "Roadside distance marker" , "Significant developmental stage" .)