Child's hero admitted to terribly heartless betrayal (9)
I believe the answer is:
treachery
'betrayal' is the definition.
(I know that betrayal can be written as treachery)
'child's hero admitted to terribly heartless' is the wordplay.
'child's hero' becomes 'Reacher' (Jack Reacher, character in books by Lee Child).
'admitted to' indicates putting letters inside.
'heartless' suggests removing the centre.
'terribly' with its centre taken out is 'ty'.
'reacher' put within 'ty' is 'TREACHERY'.
(Other definitions for treachery that I've seen before include "Perfidious behaviour" , "Betrayal of trust" , "perfidious conduct" , "Perfidy" , "Two-timing" .)