Second person with no starter to eat? Most immature (8)
I believe the answer is:
youngest
'most immature' is the definition.
('young' can be a synonym of 'immature')
'second person with no starter to eat?' is the wordplay.
'second person' becomes 'you' (I've seen this before).
'with no starter' means to remove the first letter.
'to eat?' becomes 'ingest' (eating is a kind of ingesting).
'ingest' with its first letter removed is 'ngest'.
'you'+'ngest'='YOUNGEST'
(Other definitions for youngest that I've seen before include "least experienced" , "Most youthful" , "Last-born" , "last to hatch?" , "Most junior" .)