Boy, everything’s too full-on at the end! (3)
I believe the answer is:
son
'boy' is the definition.
(male child)
'everything's too full-on at the end' is the wordplay.
I cannot quite see how this works, but
'too' could be 'so' (synonyms) and 'so' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for son that I've seen before include "The kind of child" , "Male offspring" , "Male scion" , "Human offspring" , "Express paper" .)