Vast ages (4)
I believe the answer is:
eons
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vast ages</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for eons that I've seen before include "Endless periods" , "Nose (anag)" , "Long periods of time - noes have it" , "Divisions of archaeological time" , "Divisions of geological time" .)