Parasite to run away from, by the sound of it (4)
I believe the answer is:
flea
'parasite' is the definition.
(I know that flea is a type of ectoparasite)
'run away from by the sound of it' is the wordplay.
'run away from' becomes 'flee' (I've seen this in another clue).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'flee' sounds like 'FLEA'.
'to' acts as a link.
(Other definitions for flea that I've seen before include "Irritating jumper?" , "Jumping bloodsucker" , "Blood-sucking parasites" , "tiny jumper" , "Jumping parasite" .)