Uproar of ultimately catastrophic, harmful relationship (7)
I believe the answer is:
clamour
'uproar' is the definition.
(I've seen this before)
'ultimately catastrophic harmful relationship' is the wordplay.
'ultimately' says to take the final letters.
'relationship' becomes 'amour' (amour is a kind of relationship**).
The last letters of 'catastrophic harmful' are 'cl'.
'cl'+'amour'='CLAMOUR'
'of' is the link.
(Other definitions for clamour that I've seen before include "racket" , "Hue and cry" , "Loud and persistent outcry" , "Uproar" , "Loud noise of shouting" .)