The most common and most likely answer to this crossword clue is the 13 letter word ACCOMPANIMENT
I've seen this crossword clue in The Daily Telegraph
A
C
C
O
M
P
A
N
I
M
E
N
T
Buddy explains!
The straight part
We know that "Backing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case backing = accompaniment.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 ACCOMPANIMENT mean?
ACCOMPANIMENT noun- A part, usually performed by instruments, that gives support or adds to the background in music, or adds for ornamentation; also, the harmony of a figured bass.
- That which accompanies; something that attends as a circumstance, or which is added to give greater completeness to the principal thing, or by way of ornament, or for the sake of symmetry.