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' says to put letters next to each other.
'mother' becomes 'ma' (ma can mean mother).
'to parasites' becomes 'lice' (lice are parasitic insects).
'ma' next to 'lice' is 'MALICE'.
'shown' acts as a 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" .)