The most common and most likely answer to this crossword clue is the 8 letter word FORENAME
F
O
R
E
N
A
M
E
Buddy explains!
The straight part
We know that "One is Alan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one is alan = forename.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 FORENAME mean?
FORENAME noun- A name that precedes the surname.