Wee parasite (4)
I believe the answer is:
mite
'wee parasite' 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 arachnid, person or amount of money" , "Old piece" , "Arachnid; small child" , "Tiny creature found in food" , "Microscopic creature" .)