Sharpness shown by a copper on it close to motorway (6)
I believe the answer is:
acuity
'sharpness' is the definition.
('acuity' can be a synonym of 'sharpness')
'a copper on it close to motorway' is the wordplay.
'copper' becomes 'cu' (chemical symbol**).
'on' is a charade indicator (letters next to each other).
'close to' says to take the final letters (the close/ending of).
The final letter of 'motorway' is 'y'.
'a'+'cu'+'it'+'y'='ACUITY'
'shown by' is the link.
(Other definitions for acuity that I've seen before include "Sharp-sightedness, keenness" , "Sharpness (e.g. of mind)" , "Keenness of vision" , "Mental sharpness" , "Sharpness of mind, keenness" .)