They may be largely responsible for children's nightmares (5)
I believe the answer is:
ogres
'children's nightmares' is the definition.
I can't tell whether this definition defines the answer.
'they may be largely' is the wordplay.
'they may' becomes 'og' (I can't justify this - if you can you should believe this answer much more).
'be' becomes 'rest' (resting is a kind of being).
'largely' means to remove the last letter (most but not all of the letters).
'rest' with its last letter taken away is 'res'.
'og'+'res'='OGRES'
'responsible for' is the link.
Can you help me to learn more?
(Other definitions for ogres that I've seen before include "She-giant of fairy tale" , "Giants in fairytales" , "Giant types" , "'Monsters, fiends (5)'" , "Man-eating giants of folklore" .)