Maybe a Guardian feature about German child replaced (10)
I believe the answer is:
changeling
'child replaced' is the definition.
(fairy replacing a real child)
'maybe a guardian feature about german' is the wordplay.
'maybe a guardian' becomes 'angel' (guardian angel).
'feature' becomes 'chin' (facial feature).
'about' indicates putting letters inside.
'german' becomes 'g' (abbreviation).
'angel' going within 'chin' is 'changelin'.
'changelin'+'g'='CHANGELING'
(Other definitions for changeling that I've seen before include "someone off with the fairies" , "Baby switched by fairies" , "Child secretly exchanged for another in infancy" .)