The most common and most likely answer to this crossword clue is the 5 plus 9 letter word CROSS REFERENCE
I've seen this crossword clue in Financial Times
C
R
O
S
S
R
E
F
E
R
E
N
C
E
Buddy explains!
The straight part
We know that "given by 18" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case given by 18 = cross reference.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 CROSS REFERENCE mean?
CROSS REFERENCE noun- A reference or direction in one place in a book or other source of information to information at another place in the same work
- To provide something with a cross-reference.
- To compare two or more sources of information.