No opening for bubbly host (4)
I believe the answer is:
army
'host' is the definition.
(both can mean a large number of people)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for army that I've seen before include "Military land forces" , "Large number of people" , "Fighting force" , "Fighting unit" , "A defence force" .)