Manner to compel military group (3,5)
I believe the answer is:
air force
'military group' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'manner to compel' is the wordplay.
'manner' becomes 'air' (air is a kind of manner).
'to compel' becomes 'force' (forcing is a kind of compelling).
'air'+'force'='AIR FORCE'
Can you help me to learn more?
(Other definitions for air force that I've seen before include "Country's military planes" , "Simple tune for church service" , "Kind of blue" , "one of the services" , "Armed service" .)