Sink down (3)
I believe the answer is:
sag
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sink down</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sag that I've seen before include "Cave in" , "Decline" , "Droop, flag" , "Spinach as used in Indian cookery" , "Hang down loosely" .)