Be all upset at a mere tag (5)

I believe the answer is:

label

I'm a little stuck... Click here to teach me more about this clue!

'a mere tag' is the definition.
'label' can be an answer for 'tag' (I've seen this before). I am not sure about the 'a mere' bit.

'be all upset' is the wordplay.
'upset' indicates an anagram.
'be'+'all'='beall'
'beall' anagrammed gives 'LABEL'.

'at' acts as a link.

Can you help me to learn more?

(Other definitions for label that I've seen before include "attachment to the Christmas parcel" , "Give name to" , "Tag, tab" , "Identification tag" , "Classifying name" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app