The most common and most likely answer to this crossword clue is the 10 letter word UPPER-CASE
I've seen this crossword clue in Independent
U
P
P
E
R
-
C
A
S
E
More clues leading to the result UPPER-CASE
Buddy explains!
The straight part
We know that "Capital" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case capital = upper-case.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 UPPER-CASE mean?
UPPER-CASE adjective- Written in upper case; capital