Ruin United Nations function (4)
I believe the answer is:
undo
'ruin' is the definition.
(someone's undoing is their ruin)
'united nations function' is the wordplay.
'united nations' becomes 'UN' (abbreviation).
'function' becomes 'do' (both can mean an event).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Ruin or undermine" , "Untie or ruin" , "Take apart" , "Open or unfasten" , "Dissolve - ruin" .)