The most common and most likely answer to this crossword clue is the 13 letter word PRET-A-PORTER
P
R
E
T
-
A
-
P
O
R
T
E
R
Buddy explains!
The straight part
We know that "‘off the peg’" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘off the peg’ = pret-a-porter.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.