Parasitic arachnid (4)
I believe the answer is:
mite
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">parasitic arachnid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mite that I've seen before include "Tiny contribution, child" , "Old piece" , "Microscopic creature" , "Little chap" , "Microscopic animal" .)