Shrewdness a star's shown when joining urban community (12)
I believe the answer is:
perspicacity
'shrewdness' is the definition.
(synonyms)
'a star's shown when joining urban community' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'star' becomes 'Spica' (Spica is an example).
'shown when joining' is a charade indicator (letters next to each other).
'urban community' becomes 'city'.
'per'+'spica'+'city'='PERSPICACITY'
(Other definitions for perspicacity that I've seen before include "Astute intelligence" , "Clear-mindedness - crappy cities (anag)" , "Insight, perception" , "Insight into and understanding of things" .)