Aging (9)
I believe the answer is:
senescent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aging</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that aging can be written as senescent)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for senescent that I've seen before include "Showing age-related deterioration" , "Growing old" , "Ageing" , "Showing the deterioration of age" , "Getting old" .)