Excited by the change in the air (3,2)
I believe the answer is:
het up
'excited' is the definition.
(synonyms)
'the change in the air' is the wordplay.
I cannot really see how this works, but
an anagram of 'the' is 'het' which is within the answer.
'in the air' could be 'up' and 'up' is located in the answer.
This accounts for all the letters.
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 het up that I've seen before include "Upset, agitated" , "in a state" , "Indignant" , "Angry - agitated" , "On edge - agitated" .)