Too small to be seen with the naked eye (11)
I believe the answer is:
microscopic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">too small to be seen with the naked eye</span>' is the definition.<br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for microscopic that I've seen before include "Invisible to the naked eye" , "Extremely small" , "Too small to see with the naked eye" , "Extremely tiny" .)