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