Parasitic arachnids (5)
I believe the answer is:
ticks
'parasitic arachnids' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ticks that I've seen before include "Bloodsuckers; seconds" , "Parasitic arachnids" , "Makes noise like old clock" , "Mites in mattress" , "Parasitic insects" .)