Turn in a metropolis shows sharpness (6)
I believe the answer is:
acuity
'sharpness' is the definition.
(synonyms)
'turn in a metropolis' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'in' means one lot of letters goes inside another.
'metropolis' becomes 'city' (a metropolis is a large city).
'a'+'city'='acity'
'u' placed into 'acity' is 'ACUITY'.
'shows' acts as a link.
(Other definitions for acuity that I've seen before include "Sharpness of mind, keenness" , "Mental sharpness" , "Showing ingenuity" , "Keenness of vision" , "Keenness of perception" .)