Trendy youngsters torn apart by start of unnerving nightmare (7)
I believe the answer is:
incubus
'nightmare' is the definition.
(I know that nightmare can be written as incubus)
'trendy youngsters torn apart by start of unnerving' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'youngsters' becomes 'cubs' (cub can mean a baby or child).
'torn apart by' is an insertion indicator (some letters tear an opening in others).
'start of' indicates taking the first letters.
The initial letter of 'unnerving' is 'u'.
'cubs' placed around 'u' is 'cubus'.
'in'+'cubus'='INCUBUS'
(Other definitions for incubus that I've seen before include "Male demon - nightmare - obsession" , "Nightmare - cub in us (anag)" , "Betraying Apostle" , "Demon bringing nightmares" , "Oppressive burden" .)