Someone else a hornet upset (7)
I believe the answer is:
another
'someone else' is the definition.
(I've seen this before)
'a hornet upset' is the wordplay.
'upset' indicates anagramming the letters.
'hornet' with letters rearranged gives 'nother'.
'a'+'nother'='ANOTHER'
(Other definitions for another that I've seen before include "A further (item)" , "Additional, one more" , "second" , "No heart (anag.)" , "A different one" .)