I’m upset, struggling for momentum (7)
I believe the answer is:
impetus
'momentum' is the definition.
(impetus can mean momentum or motivation)
'i'm upset struggling' is the wordplay.
'i'm' becomes 'im' ().
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'upset' is an anagram of 'petus'.
'im'+'petus'='IMPETUS'
'for' is the link.
(Other definitions for impetus that I've seen before include "Drive" , "Incentive - time's up (anag)" , "Thrust" , "Catalyst" , "Force that moves something along" .)