Relative embracing sources of child's immunisation for young member of the family (5)
I believe the answer is:
scion
'young member of the family' is the definition.
'relative embracing sources of child's immunisation' is the wordplay.
'relative' becomes 'son' (son is a kind of relative**).
'embracing' indicates putting letters inside.
'sources of' suggests taking the first letters.
The initial letters of 'childs immunisation' is 'ci'.
'son' enclosing 'ci' is 'SCION'.
'for' is the link.
(Other definitions for scion that I've seen before include "Offspring" , "Young shoot/twig" , "Young shoot or twig" , "Plant shoot; descendant of a noble family;" , "Descendent - heir" .)