In one's mind (8)
I believe the answer is:
mentally
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">in one's mind</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mentally that I've seen before include "not in writing" , "in the head" , "by way of intellect" , "in their heads" , "Cerebrally" .)