Flat ring (6)
I believe the answer is:
washer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flat ring</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for washer that I've seen before include "Rendered sightless" , "keep things firm" , "Laundress" , "Metal disc for hawser" , "Tap seal" .)