Responds again to the "Scat!" by going off (6)
I believe the answer is:
reacts
'responds' is the definition.
('react' can be a synonym of 'respond')
'again to the scat by going off' is the wordplay.
I cannot quite see how this works, but
'again' could be 're' (prefix meaning 'again') and 're' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for reacts that I've seen before include "Responds to stimulus (6)" , "Acts in response to something" , "Responds to an impulse" , "Retaliates" .)