Mark correct disease vector (4)
I believe the answer is:
tick
I believe this is a double definition.
'mark correct' is the first definition.
(to give an answer a tick is to mark it correct)
'disease vector' is the second definition.
(tick is an infectious arachnid)
(Other definitions for tick that I've seen before include "Parasitic insect or clicking sound" , "delayed payment" , "Sound of an old watch" , "Parasitic insect or pillow casing" , "Small parasitic arachnid" .)