Host is shot when it gets a bit cooler (5)
I believe the answer is:
swarm
'host' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for swarm that I've seen before include "Group of social insects" , "Find insect population" , "Plague of insects" , "Climb; cluster of bees" , "Moving crowd, of bees say" .)