Onion ring one boy consumes (8)
I believe the answer is:
scallion
'onion' is the definition.
(scallion is a kind of onion)
'ring one boy consumes' is the wordplay.
'ring' becomes 'call' (synonyms).
'one' becomes 'I' (Roman numeral).
'boy' becomes 'son' (synonyms).
'consumes' is an insertion indicator.
'call'+'i'='calli'
'calli' inserted into 'son' is 'SCALLION'.
(Other definitions for scallion that I've seen before include "sort of onion" , "Green onion" , "Vegetable" , "A spring onion with long neck" , "Spring 12" .)