What could be mooniest feelings (8)
I believe the answer is:
emotions
'feelings' is the definition.
(emotion is a kind of feeling)
'what could be mooniest' is the wordplay.
'what could be' is an anagram indicator.
'mooniest' anagrammed gives 'EMOTIONS'.
(Other definitions for emotions that I've seen before include "Feelings provoked by noise, Tom" , "Intense mental feelings, like love or hate" , "Passions" , "Strong feelings" , "instinctive feelings" .)