Demons invoked by popular youngster on island (6)
I believe the answer is:
incubi
'demons' is the definition.
(incubus is a kind of demon)
'popular youngster on island' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'youngster' becomes 'cub' (cub can mean a young person).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'island' becomes 'i' (geographical abbreviation).
'in'+'cub'+'i'='INCUBI'
'invoked by' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for incubi that I've seen before include "nightmares" , "Demons of the night" .)