Tiny arachnid (4)
I believe the answer is:
mite
'tiny arachnid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mite that I've seen before include "Old piece" , "Tiny arachnid, person or amount of money" , "The widow's contribution, just in time" , "Microscopic creature" , "Minute arachnid" .)