Completely upset by terrible threats (7)
I believe the answer is:
shatter
'completely upset' is the definition.
(I've seen this before)
'terrible threats' is the wordplay.
'terrible' is an anagram indicator.
'threats' with letters rearranged gives 'SHATTER'.
'by' is the link.
(Other definitions for shatter that I've seen before include "Break into many bits" , "total" , "Smash to pieces" , "Ruin" , "Threats (anag.)" .)