Lowest possible level (4,6)
I believe the answer is:
rock bottom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lowest possible level</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 rock bottom that I've seen before include "Hard bed" , "As low as you can get" , "worst condition possible" , "such a price is heavily reduced" , "Lowest possible level" .)