Family heir required to eat every bit of spring onion (8)
I believe the answer is:
scallion
'spring onion' is the definition.
(I've seen this before)
'family heir required to eat every bit' is the wordplay.
'family heir' becomes 'scion' (a scion is a descendant of an important family).
'required to eat' is an insertion indicator.
'every bit' becomes 'all' (all of it).
'scion' enclosing 'all' is 'SCALLION'.
'of' acts as a link.
(Other definitions for scallion that I've seen before include "Green onion" , "Spring 12" , "A spring onion with long neck" , "sort of onion" , "Vegetable" .)