Was upsetting jolly throng (5)
I believe the answer is:
swarm
'throng' is the definition.
(thesaurus)
'was upsetting jolly' is the wordplay.
'upsetting' indicates anagramming the letters.
'jolly' becomes 'rm'.
'was' with letters rearranged gives 'swa'.
'swa'+'rm'='SWARM'
(Other definitions for swarm that I've seen before include "Move in large numbers like locusts" , "Body of bees" , "Plague of insects" , "Find insect population" , "Horde" .)