Figures out one is upset by poison (5)
I believe the answer is:
venom
'poison' is the definition.
(venom is a kind of poison)
'figures out one is upset' is the wordplay.
I cannot quite see how this works, but
an anagram of 'one' is 'eno' which is present in the answer.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for venom that I've seen before include "Snake's poison" , "Poison - spite" , "acrimony" , "Virulent feeling" , "Poison; malice" .)