Secret group bound to be reduced (11)
I believe the answer is:
clandestine
'secret' is the definition.
('clandestine' can be a synonym of 'secret')
'group bound to be reduced' is the wordplay.
'group' becomes 'clan' (clan is a kind of group**).
'bound to' becomes 'destined' ('destined' can be a synonym of 'bound to').
'be reduced' means to remove the last letter.
'destined' with its final letter removed is 'destine'.
'clan'+'destine'='CLANDESTINE'
(Other definitions for clandestine that I've seen before include "Concealed" , "Surreptitious" , "Private" , "Done secretively" , "backroom" .)