The most common and most likely answer to this crossword clue is the 10 letter word SUPPLEMENT
I've seen this crossword clue in The Guardian
S
U
P
P
L
E
M
E
N
T
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 = supplement.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 SUPPLEMENT mean?
SUPPLEMENT noun- Something added, especially to make up for a deficiency.
- An extension to a document or publication that adds information, corrects errors or brings up to date.
- An additional section of a newspaper devoted to a specific subject.
- An angle that, when added to a given angle, makes 180°; a supplementary angle.
- (nutrition) A vitamin, herbal extract or chemical compound ingested to meet dietary deficiencies or enhance muscular development.
- An additional charge, especially for food in a restaurant.
- To provide or make a supplement to something.