Secret agents capturing city group (7)
I believe the answer is:
species
'group' is the definition.
(eg group of animals)
'secret agents capturing city' is the wordplay.
'secret agents' becomes 'spies' (spy is a kind of secret agent).
'capturing' is an insertion indicator.
'city' becomes 'EC' (the City of London postcode).
'spies' going around 'ec' is 'SPECIES'.
(Other definitions for species that I've seen before include "Living group; kind" , "Sort, type" , "Category" , "Kind, sort" , "Biological grouping" .)