The most common and most likely answer to this crossword clue is the 8 letter word SLAPHEAD
I've seen this crossword clue in Financial Times
S
L
A
P
H
E
A
D
Buddy explains!

The straight part
We know that "One perhaps requiring a 15 across" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one perhaps requiring a 15 across = slaphead.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 SLAPHEAD mean?
SLAPHEAD noun- A bald person; somebody with a shaved or hairless head
- Any individual who annoys and frustrates others to the point of making them want to slap their own heads.