I'm terribly upset with Momentum (7)
I believe the answer is:
impetus
'momentum' is the definition.
(I've seen this before)
'i'm terribly upset' is the wordplay.
'i'm' becomes 'im' ().
'terribly' is an anagram indicator.
'upset' anagrammed gives 'petus'.
'im'+'petus'='IMPETUS'
'with' is the link.
(Other definitions for impetus that I've seen before include "influence" , "Moving force" , "Driving force, forward movement" , "Driving force, momentum" , "Motivation" .)