Celebration of artificial intelligence encountering alien in outer galaxy (6)
I believe the answer is:
gaiety
'celebration' is the definition.
(thesaurus)
'artificial intelligence encountering alien in outer galaxy' is the wordplay.
'artificial intelligence' becomes 'AI' (synonyms).
'encountering' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extra terrestrial).
'in' is an insertion indicator.
'outer' means to remove the middle letters.
'galaxy' with its middle removed is 'gy'.
'ai'+'et'='aiet'
'aiet' put into 'gy' is 'GAIETY'.
'of' is the link.
(Other definitions for gaiety that I've seen before include "Light-hearted state" , "Cheerfulness - merrymaking" , "Light hearted state" , "Festive merry feeling" , "Good humour" .)