Decline, deteriorate (6)
I believe the answer is:
worsen
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">decline</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that decline can be written as worsen)</span><br/ ><br/ >'<span class="explanation-format__original">deteriorate</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that deteriorate is a more specific form of the action the worsen)</span><br/ ><br/ ></span>
(Other definitions for worsen that I've seen before include "Deteriorate for owners" , "Become more inferior" , "go downhill" , "Go to pot" , "Become less well" .)