Foster child's fuss disturbed Pete (7)
I believe the answer is:
adoptee
'foster child's' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'fuss disturbed pete' is the wordplay.
'fuss' becomes 'ado' ('ado' can be a synonym of 'fuss').
'disturbed' indicates an anagram.
'pete' is an anagram of 'ptee'.
'ado'+'ptee'='ADOPTEE'
Can you help me to learn more?
(Other definitions for adoptee that I've seen before include "One taken on by new parents" , "one's taken into the family" , "Child taken on by another family" , "one who's left home" .)