Yon guest thrown out as most immature (8)
I believe the answer is:
youngest
'most immature' is the definition.
('young' can be a synonym of 'immature')
'yon guest thrown out' is the wordplay.
'thrown out' is an anagram indicator.
'yon'+'guest'='yonguest'
'yonguest' with letters rearranged gives 'YOUNGEST'.
'as' is the link.
(Other definitions for youngest that I've seen before include "baby" , "Most youthful" , "delivered most recently" , "Ath the earliest stage of development" , "last to hatch?" .)