Minute parasite (4)
(Other definitions for tick that I've seen before include "Minute" , "Insect; noise of watch" , "Parasite, moment (colloq.)" , "Credit - bloodsucker" , "Credit; insect" .)
(Other definitions for mite that I've seen before include "Little chap" , "Minute arachnid" , "Old piece" , "Microscopic creature" , "Minute creature" .)