Signs of parasites (5)
I believe the answer is:
ticks
This clue is a double definition.
'signs' is the first definition.
(I have seen 'Signs of approval ' mean 'ticks' so perhaps 'signs' could also mean 'ticks')
'parasites' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for ticks that I've seen before include "Parasites" , "Marks correct" , "Bloodsuckers; seconds" , "Marks right" , "Parasitic insects" .)