Grow worse (11)
I believe the answer is:
deteriorate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grow worse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(deteriorating is a kind of growing worse)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for deteriorate that I've seen before include "Gradually worsen" , "Become progressively worse" , "Grow worse" , "Go downhill" , "Fail" .)