The most common and most likely answer to this crossword clue is the 13 letter word CONCATENATION
I've seen this crossword clue in The Daily Telegraph
C
O
N
C
A
T
E
N
A
T
I
O
N
Buddy explains!
The straight part
We know that "chain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chain = concatenation.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 CONCATENATION mean?
CONCATENATION noun- A series of links united; a series or order of things depending on each other, as if linked together; a chain, a succession.
- The application of these series of links.
- The operation of joining multiple character strings.
- A character string formed by joining multiple character strings.