The dog, the time after, got upset by the poison (6)
I believe the answer is:
curare
'the poison' is the definition.
(I've seen this before)
'the dog the time after got upset' is the wordplay.
I cannot quite see how this works, but
'dog' could be 'cur' (both can mean a despicable person) and 'cur' is found in the answer.
The remaining letters 'are' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for curare that I've seen before include "Poison applied to darts" , "Arrow poison" , "Poison used on darts" , "Paralysing plant poison" , "Poison put on arrows" .)