Granny joins group going round French city (6)
I believe the answer is:
nantes
'french city' is the definition.
(Nantes is a city in France)
'granny joins group going round' is the wordplay.
'granny' becomes 'nan'.
'joins' is a charade indicator (letters next to each other).
'group' becomes 'set' (both can mean a collection).
'going round' shows that the letters should be reversed in order.
'set' in reverse letter order is 'tes'.
'nan'+'tes'='NANTES'
(Other definitions for nantes that I've seen before include "French FC" , "French football club" , "A major French port" , "Port city in western France" , "Ann set out for the French town" .)