Native language (6,6)
I believe the answer is:
mother tongue
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">native language</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mother tongue that I've seen before include "Native language" , "Language used" .)