On a high, gush about wonderful day (6,3)
I believe the answer is:
spaced out
'on a high' is the definition.
I can't judge whether this definition defines the answer.
'gush about wonderful day' is the wordplay.
'gush' becomes 'spout' (synonyms).
'about' is an insertion indicator.
'wonderful' becomes 'ace' (ace can informally mean very good).
'day' becomes 'd' (abbreviation for day).
'ace'+'d'='aced'
'spout' going around 'aced' is 'SPACED-OUT'.
Can you help me to learn more?
(Other definitions for spaced out that I've seen before include "Disorientated through drugs" , "Separated" , "on a trip" , "in a dazed state" , "Not quite all there?" .)