City's main "Mr. Big" busted importing heroin (10)
I believe the answer is:
birmingham
'city's' is the definition.
(Birmingham is an example)
'main mr big busted importing heroin' is the wordplay.
'busted' indicates an anagram.
'importing' is an insertion indicator.
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'main'+'mr'+'big'='mainmrbig'
'mainmrbig' anagrammed gives 'birmingam'.
'birmingam' going around 'h' is 'BIRMINGHAM'.
(Other definitions for birmingham that I've seen before include "Aston Villa's local rivals, the Blues" , "Second largest UK city" , "Brum" , "England's second biggest city" .)