Hostility shown by parent towards parasites (6)
I believe the answer is:
malice
'hostility' is the definition.
(thesaurus)
'parent towards parasites' is the wordplay.
'parent' becomes 'ma' (ma can mean mum or mother).
'towards' says to put letters next to each other (I've seen this in other clues).
'parasites' becomes 'lice' (lice are parasitic insects).
'ma'+'lice'='MALICE'
'shown by' is the link.
(Other definitions for malice that I've seen before include "Desire to harm others, may be aforethought" , "Hate" , "'Ill-will, spite (6)'" , "Hatred" , "Ill will, a desire to harm others" .)