Not initially longing to accept guardian spirit or substitute (10)
I believe the answer is:
changeling
'substitute' is the definition.
(thesaurus)
'not initially longing to accept guardian spirit' is the wordplay.
'not initially' means to remove the first letter.
'longing' becomes 'aching' ('ache' can be a synonym of 'long').
'to accept' is an insertion indicator.
'guardian spirit' becomes 'angel' (I am not sure about the 'guardian' bit.**).
'aching' with its initial letter taken away is 'ching'.
'ching' placed around 'angel' is 'CHANGELING'.
'or' is the link.
Can you help me to learn more?
(Other definitions for changeling that I've seen before include "Child secretly exchanged for another in infancy" , "Baby switched by fairies" , "someone off with the fairies" .)