The most common and most likely answer to this crossword clue is the 5 plus 4 letter word FIRST NAME
I've seen this crossword clue in The Guardian
F
I
R
S
T
N
A
M
E
Buddy explains!
The straight part
We know that "It could be William" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it could be william = first name.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 FIRST NAME mean?
FIRST NAME noun- The first element of a full name in cultures that place the given name first.