Mary upset the men (4)
I believe the answer is:
army
'the men' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Maybe there's an association between them I don't understand?
'mary upset' is the wordplay.
'upset' indicates anagramming the letters.
'mary' is an anagram of 'ARMY'.
Can you help me to learn more?
(Other definitions for army that I've seen before include "A defence force" , "Collective term for caterpillars" , "Body of people organised for war" , "Large military body" , "A great number (of soldiers?)" .)