Distance from motorway, Alex discovered? (4)
I believe the answer is:
mile
'distance' is the definition.
'motorway alex discovered?' is the wordplay.
'motorway' becomes 'MI' (resembles M1, English motorway).
'discovered?' says to take the centre (I've seen 'discovered' mean this (dis-covered - outer letters removed)).
The centre of 'alex' is 'le'.
'mi'+'le'='MILE'
'from' acts as a link.
(Other definitions for mile that I've seen before include "Distance race" , "Unit of distance" , "Imperial length" , "'1,760 yards (4)'" , "Unit of length" .)