The most common and most likely answer to this crossword clue is the 10 letter word CONCLUSIVE
I've seen this crossword clue in The Guardian
C
O
N
C
L
U
S
I
V
E
Buddy explains!

The straight part
We know that "emphatic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case emphatic = conclusive.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CONCLUSIVE mean?
CONCLUSIVE adjective- Pertaining to a conclusion.
- Providing an end to something; decisive.