It's dismissive in a scathing way (4)
I believe the answer is:
scat
'way' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for scat that I've seen before include "leave quickly" , "Bolt" , "One way of singing" , "Dropping" , "Jazz singing - animal droppings - silvery fish" .)