Parasites making right marks (5)
I believe the answer is:
ticks
'parasites' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ticks that I've seen before include "covers" , "Marks correct" , "Marks of approval" , "They suck from warm-blooded animals" , "Signs of approval" .)