The most common and most likely answer to this crossword clue is the 10 letter word ADDITIONAL
I've seen this crossword clue in The Guardian
A
D
D
I
T
I
O
N
A
L
More clues leading to the result ADDITIONAL
Buddy explains!

The straight part
We know that "Extra" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extra = additional.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 ADDITIONAL mean?
ADDITIONAL noun- Something added.
- Supplemental or added to something.