Decreases in strength (5)
I believe the answer is:
wanes
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">decreases in strength</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for wanes that I've seen before include "Gradually declines" , "Decreases in vigour or strength like the moon" , "Of the moon, gradually decreases" , "Grows smaller like the moon" .)