Host was preparing room (5)
I believe the answer is:
swarm
'host' is the definition.
(I've seen this before)
'was preparing room' is the wordplay.
'preparing' indicates anagramming the letters (the same letters prepared differently).
'room' becomes 'rm' (I've seen this in other clues).
'was' is an anagram of 'swa'.
'swa'+'rm'='SWARM'
(Other definitions for swarm that I've seen before include "Climb; cluster of bees" , "Move in large numbers like locusts" , "Large group of flying insects" , "Plague of insects" , "Moving crowd, of bees say" .)