Hosts upset by a miser (6)
I believe the answer is:
armies
'hosts' is the definition.
(I've seen this in another clue)
'upset by a miser' is the wordplay.
'upset by' indicates anagramming the letters.
'a'+'miser'='amiser'
'amiser' anagrammed gives 'ARMIES'.
(Other definitions for armies that I've seen before include "Bodies of people organised for war" , "Fighting forces" , "Multitudes" , "Organizations of military land forces" , "Hosts" .)