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' (something that's 'in' is fashionable).
'youngster' becomes 'cub' (cub can mean a young person).
'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" .)