Group meeting fate underground (11)
I believe the answer is:
clandestine
'underground' is the definition.
(synonyms)
'group meeting fate' is the wordplay.
'group' becomes 'clan' (clan is a kind of group).
'meeting' means one lot of letters go next to another.
'fate' becomes 'destine' (to be fated for something is to be destined for it).
'clan'+'destine'='CLANDESTINE'
(Other definitions for clandestine that I've seen before include "Surreptitious" , "Done secretively" , "Undercover" , "backroom" , "Done secretly" .)