Clandestine group limiting City resistance (6)
I believe the answer is:
secret
'clandestine' is the definition.
(I know that clandestine can be written as secret)
'group limiting city resistance' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'limiting' means one lot of letters goes inside another.
'city' becomes 'EC' (the City of London postcode).
'resistance' becomes 'r' (electronics abbreviation).
'ec'+'r'='ecr'
'set' placed around 'ecr' is 'SECRET'.
(Other definitions for secret that I've seen before include "Unknown or unobserved by others" , "Unidentified" , "Private, covert" , "Confidential, not to be revealed" , "Hidden; unknown" .)