Sinister body regulating ads in the City (5)
I believe the answer is:
lhasa
'ads in the city' is the definition.
'lhasa' can be an answer for 'city' (Lhasa is an example). I'm not certain of the 'ads in the' bit.
'sinister body regulating' is the wordplay.
'sinister' becomes 'L' (short for left - 'sinister' is 'left' in Latin).
'body regulating' becomes 'hasa' (I can't explain this - if you can you should believe this answer much more).
'l'+'hasa'='LHASA'
Can you help me to learn more?
(Other definitions for lhasa that I've seen before include "Sacred city" , "Capital of Tibet" , "Asian capital" , "Tibetan holy city" , "foreign capital" .)