A trio will upset me, so there! (9)
I believe the answer is:
threesome
'a trio' is the definition.
(I know that trio can be written as threesome)
'upset me so there' is the wordplay.
'upset' indicates anagramming the letters.
'me'+'so'+'there'='mesothere'
'mesothere' with letters rearranged gives 'THREESOME'.
'will' is the link.
(Other definitions for threesome that I've seen before include "triangular event" , "Chekhov's sisters" , "Small group of people" , "Blind mice, perhaps" , "Horse meet (anag.)" .)