Trend initially used by City Group (4)
I believe the answer is:
trio
'group' is the definition.
(a group of three)
'trend 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 initial letter of 'trend' is 't'.
't'+'rio'='TRIO'
(Other definitions for trio that I've seen before include "Group of three (musicians)" , "Beethoven's 'Archduke': famous piece written for this ensemble" , "Small group playing" , "Archduke, possibly" , "Beethoven's 'Archduke', for example" .)