Paving slab (9)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">paving slab</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 flagstone that I've seen before include "Fine-textured rock" , "One walked on" , "Pavior made from Teflon gas" , "Paving slab" , "It'll lay (some say) at ground level" .)
(Other definitions for flag that I've seen before include "National standard" , "Emblematic rectangular piece of fabric" , "what Amundsen planted" , "Iris - paving slab" , "sink" .)