The most common and most likely answer to this crossword clue is the 8 plus 5 letter word LAVATORY PAPER
I've seen this crossword clue in The Guardian
L
A
V
A
T
O
R
Y
P
A
P
E
R
Buddy explains!
The straight part
We know that "That’s the Daily Mail to Guardian readers! /That’s the Daily Mail to Guardian readers!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s the daily mail to guardian readers! /that’s the daily mail to guardian readers! = lavatory paper.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.