Youth essentially brightening poet's evening (4)
I believe the answer is:
teen
'youth' is the definition.
(teenager)
'essentially brightening poet's evening' is the wordplay.
'essentially' indicates the central letters.
'poet's evening' becomes 'e'en' (I've seen this in another clue).
The central letter of 'brightening' is 't'.
't'+'een'='TEEN'
(Other definitions for teen that I've seen before include "Youngster of a certain age" , "Young person" , "Youth" , "Adolescent, then adult-to-be" , "Juvenile" .)