Pollution choking city guide (6)
I believe the answer is:
direct
'guide' is the definition.
(I know that guide can be written as direct)
'pollution choking city' is the wordplay.
'pollution' becomes 'dirt' (pollution can mean dirt or mess).
'choking' means one lot of letters goes inside another (inserted letters are choked or crushed).
'city' becomes 'EC' (the City of London postcode).
'dirt' placed around 'ec' is 'DIRECT'.
(Other definitions for direct that I've seen before include "Guide with credit" , "Outspoken" , "Frank; unambiguous" , "Point the way" , "Take charge, of a company or film say" .)