Nervously excited mostly with treat being arranged (8)
I believe the answer is:
atwitter
'nervously excited' is the definition.
'atwitter' can be an answer for 'excited' (atwitter can mean in a state of excitement). I am not sure about the 'nervously' bit.
'mostly with treat being arranged' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'being arranged' is an anagram indicator (letters rearranged).
'with' with its last letter taken off is 'wit'.
'wit'+'treat'='wittreat'
'wittreat' is an anagram of 'ATWITTER'.
Can you help me to learn more?
(Other definitions for atwitter that I've seen before include "Noisy" , "very chirpy?" , "excited" .)