To horrify dad is wrong, mate (5)
I believe the answer is:
appal
'to horrify' is the definition.
('appal' can be a synonym of 'horrify')
'dad is wrong mate' is the wordplay.
I cannot really see how this works, but
'dad' could be 'pa' (both can mean one's father) and 'pa' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for appal that I've seen before include "Fill with horror" , "Shock, horrify" , "Cause to be unpleasantly surprised" , "exhibiting dismay" , "Outrage" .)