After one short month, they get Rosie no end upset! (7)

I believe the answer is:

juniors

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'after one short month they' is the definition.
The definition suggests a plural noun which matches the answer.

'get rosie no end upset' is the wordplay.
'get' becomes 'jun' (I can't explain this - if you can you should believe this answer much more).
'no end' means to remove the last letter.
'upset' indicates anagramming the letters.
'rosie' with its final letter removed is 'rosi'.
'rosi' anagrammed gives 'iors'.
'jun'+'iors'='JUNIORS'

Can you help me to learn more?

(Another definition for juniors that I've seen is " They're lower in rank".)

I've seen this clue in the Evening Standard.

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