Ill-will shown by mother to parasites (6)
I believe the answer is:
malice
'ill-will' is the definition.
(I've seen this before)
'by mother to parasites' is the wordplay.
'by' is a charade indicator (letters next to each other).
'mother' becomes 'ma' (ma can mean mother).
'to parasites' becomes 'lice' (lice are parasitic insects).
'ma'+'lice' is 'MALICE'.
'shown' is the link.
(Other definitions for malice that I've seen before include "Spite and rancour" , "Spite - desire to hurt" , "Wrongful intention" , "Spiteful, animosity, sometimes a forethought" , "Spitefulness" .)