Getting on after mother is prejudicial (8)
I believe the answer is:
damaging
'prejudicial' is the definition.
('damaging' can be a synonym of 'prejudicial')
'getting on after mother' is the wordplay.
'getting on' becomes 'aging' (I've seen this before).
'after' says to put letters next to each other.
'mother' becomes 'dam' (female parent of an animal).
'aging' after 'dam' is 'DAMAGING'.
'is' is the link.
(Other definitions for damaging that I've seen before include "Hurting" , "injurious" , "Causing harm or injury" .)