Mary upset the men (4)
I believe the answer is:
army
'the men' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'mary upset' is the wordplay.
'upset' indicates an anagram.
'mary' anagrammed gives '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?)" .)