Eternity, vast age (3)
I believe the answer is:
eon
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">eternity</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">vast age</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for eon that I've seen before include "plenty of time" , "Very long age" , "Vast age; eternity" , "Immense period" , "Major division of geological time" .)