The most common and most likely answer to this crossword clue is the 8 letter word COGNOMEN
I've seen this crossword clue in The Times
C
O
G
N
O
M
E
N
Buddy explains!

The straight part
We know that "Nickname" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nickname = cognomen.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 COGNOMEN mean?
COGNOMEN noun- Surname.
- The third part of the name of a citizen of Ancient Rome.
- A nickname or epithet by which someone is identified.