Parasitic arachnid (4)
I believe the answer is:
mite
'parasitic arachnid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mite that I've seen before include "Tiny contribution, child" , "Old piece" , "Microscopic creature" , "Little chap" , "Microscopic animal" .)