The most common and most likely answer to this crossword clue is the 9 letter word OAK-APPLE
I've seen this crossword clue in The Times
O
A
K
-
A
P
P
L
E
Buddy explains!
The straight part
We know that "The gall!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the gall! = oak-apple.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.