French mark (7)
I believe the answer is:
cedilla
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">french mark</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 cedilla that I've seen before include "mark not seen in 18 [FACADE]?" , "Small mark placed below C in Portuguese and Catalan, for example" , "for French character" , "Sign to soften C" , "Diacritical mark" .)