The most common and most likely answer to this crossword clue is the 4 letter word YAPP
Y
A
P
P
Buddy explains!
The straight part
We know that "Type of binding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case type of binding = yapp.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 YAPP mean?
YAPP noun- In bookbinding, the edge of the binding that extends beyond the edges of the pages, and may protect the gilding.