Mark, irritating little thing (4)
I believe the answer is:
tick
I believe this clue is a double definition.
'mark' is the first definition.
(I know that tick is a type of mark)
'irritating little thing' is the second definition.
(irritating parasite)
(Other definitions for tick that I've seen before include "Small parasitic arachnid" , "Credit (sl.)" , "Clock's sound" , "Mark of work done (right)" , "Credit (slang)" .)