One's ruined for ages (4)
I believe the answer is:
eons
'ages' is the definition.
(I know this)
'one's ruined' is the wordplay.
'ruined' indicates an anagram.
'ones' is an anagram of 'EONS'.
'for' is the link.
(Other definitions for eons that I've seen before include "Very long periods (US)" , "Endless periods" , "Divisions of geological time" , "Long periods of time - noes have it" , "Vast ages" .)