One's working for ages (4)
I believe the answer is:
eons
'for ages' is the definition.
(eon is a kind of age)
'one's working' is the wordplay.
'working' indicates anagramming the letters.
'ones' is an anagram of 'EONS'.
(Other definitions for eons that I've seen before include "Divisions of archaeological time" , "Divisions of geological time" , "Times" , "Years and years" , "Long periods of time - noes have it" .)