Ruined when performed by the United Nations (6)
I believe the answer is:
undone
'ruined' is the definition.
(I've seen this before)
'performed by the united nations' is the wordplay.
'performed' becomes 'done' (I've seen this before**).
'by' means one lot of letters go next to another.
'the united nations' becomes 'un' ('UN' can be a synonym of 'united nations').
'done' put after 'un' is 'UNDONE'.
'when' is the link.
(Other definitions for undone that I've seen before include "Not finished; ruined" , "Reversed" , "Brought to ruin or buttons not fastened" , "Ruined - no longer fastened" , "Not finished; untied" .)