Pat is evil when knocked back (3)
I believe the answer is:
dab
'pat' is the definition.
(I know that pat can be written as dab)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dab that I've seen before include "Apply small amounts (usually a liquid) to a surface" , "Expert; flatfish" , "A bad sort of flatfish" , "Apply light touches of colour to the fish" , "Press gently; flatfish" .)