Getting older (5)
I believe the answer is:
aging
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">getting older</span>' is the definition.<br/ ><span class="explanation-format__explanation">(aging means getting older)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for aging that I've seen before include "Growing old in the US" , "in decline" , "Becoming more grown-up" , "development" , "Getting older" .)