Upset entering dark early class (12)
I believe the answer is:
kindergarten
'early class' is the definition.
(school for young children)
'upset entering dark' is the wordplay.
'upset' is an anagram indicator.
'entering'+'dark'='enteringdark'
'enteringdark' with letters rearranged gives 'KINDERGARTEN'.
(Other definitions for kindergarten that I've seen before include "Where master may be" , "minor institution" , "Pre-school institution for little children" , "Children's school" , "Pre-primary school class" .)