Be all upset by a tag (5)
I believe the answer is:
label
'a tag' is the definition.
(I've seen this before)
'be all upset' is the wordplay.
'upset' is an anagram indicator.
'be'+'all'='beall'
'beall' with letters rearranged gives 'LABEL'.
'by' acts as a link.
(Other definitions for label that I've seen before include "Means of identifying" , "Identification tag" , "Classifying name" , "Identify" , "Identity tag" .)