Group of a hundred in the city (5)
I believe the answer is:
batch
'group' is the definition.
(I know this)
'a hundred in the city' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'in the' indicates putting letters inside.
'city' becomes 'bath' (English city).
'c' placed within 'bath' is 'BATCH'.
'of' is the link.
(Other definitions for batch that I've seen before include "Quantity of goods produced" , "Group of objects prepared or sent together" , "Consignment of goods" , "All the bread baked at the same time" , "All the loaves baked at the same time" .)