Tactic initially used by City Group (4)
I believe the answer is:
trio
'group' is the definition.
(a group of three)
'tactic initially used by city' is the wordplay.
'initially' says to take the initial letters.
'used by' means one lot of letters go next to another.
'city' becomes 'rio' (Rio de Janeiro).
The first letter of 'tactic' is 't'.
't'+'rio'='TRIO'
(Other definitions for trio that I've seen before include "Ensemble musical piece, eg Beethoven's 'Archduke'" , "Chamber group" , "(Composition for) three performers" , "Archduke, possibly" , "A group or set of three" .)