Pet dragon repaired by one supposed to have child's interests at heart (9)
I believe the answer is:
godparent
'one supposed to have child's interests at heart' is the definition.
(a godparent guides a child through life)
'pet dragon repaired' is the wordplay.
'repaired' is an anagram indicator.
'pet'+'dragon'='petdragon'
'petdragon' anagrammed gives 'GODPARENT'.
'by' is the link.
(Other definitions for godparent that I've seen before include "Baptism sponsor" , "Pet dragon (anag)" , "Religious guardian" , "as one responsible for charge" .)