Worsen (10)
There are a few possibilities for this:
degenerate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">worsen</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 relapse that I've seen before include "Fall ill again after rallying" , "Fall back into old ways" , "setback in health terms" , "Pleaser may slip to earlier state" , "Get worse again" .)
(Other definitions for decline that I've seen before include "Refuse; decay" , "Start to fail" , "Diminution" , "The Latin scholar must" , "Evelyn Waugh novel" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">worsen</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that degenerate is a kind of the worsen)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for degenerate that I've seen before include "Become worse than before" , "Debauched" , "Mean" , "Immoral" , "dishonourable" .)