Popular youngster primarily interpreting nightmares (6)
I believe the answer is:
incubi
'nightmares' is the definition.
('incubus' can be a synonym of 'nightmare')
'popular youngster primarily interpreting' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'youngster' becomes 'cub' (young bear).
'primarily' says to take the initial letters.
The first letter of 'interpreting' is 'i'.
'in'+'cub'+'i'='INCUBI'
(Other definitions for incubi that I've seen before include "nightmares" , "Demons of the night" .)