The most common and most likely answer to this crossword clue is the 7 plus 5 letter word CONCERT PITCH
I've seen this crossword clue in The Guardian
C
O
N
C
E
R
T
P
I
T
C
H
Buddy explains!
The straight part
We know that "International standard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case international standard = concert pitch.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.