Group of spies mass surreptitiously? (6,7)
I believe the answer is:
secret service
This might be a double definition.
'group of spies' is the first definition.
'secret service' can be an answer for 'spies' (I've seen this before). I'm not certain of the 'group of' bit.
'mass surreptitiously?' is the second definition.
The definition and answer can be both groups as well as being singular nouns.
Maybe they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for secret service that I've seen before include "Government agency" , "Spies' department" , "MI5 or MI6 (informal)" , "Spying agency" , "Government department involved in espionage" .)