The most common and most likely answer to this crossword clue is the 7 letter word BINDING
I've seen this crossword clue in The Daily Telegraph
B
I
N
D
I
N
G
Buddy explains!
The straight part
We know that "Compulsory/cover" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case compulsory/cover = binding.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 BINDING mean?
BINDING verb- To tie; to confine by any ligature.
- To cohere or stick together in a mass.
- To be restrained from motion, or from customary or natural action, as by friction.
- To exert a binding or restraining influence.
- To tie or fasten tightly together, with a cord, band, ligature, chain, etc.
- To confine, restrain, or hold by physical force or influence of any kind.
- To couple.
- To oblige, restrain, or hold, by authority, law, duty, promise, vow, affection, or other social tie.
- To put (a person) under definite legal obligations, especially, under the obligation of a bond or covenant.
- To place under legal obligation to serve.
- To protect or strengthen by applying a band or binding, as the edge of a carpet or garment.
- To make fast (a thing) about or upon something, as by tying; to encircle with something.
- To cover, as with a bandage.
- To prevent or restrain from customary or natural action, as by producing constipation.
- To put together in a cover, as of books.
- To make two or more elements stick together.
- To associate an identifier with a value; to associate a variable name, method name, etc. with the content of a storage location.
- To complain; to whine about something.
- An item (usually rope, tape, or string) used to hold two or more things together.
- The spine of a book where the pages are held together.
- A finishing on a seam or hem of a garment.
- The association of a named item with an element of a program.
- The interface of a library with a programming language other than one it is written in.
- The action or result of making two or more molecules stick together.
- (of an agreement, contract, etc.) Imposing stipulations or requirements that must be honoured.
- (of food) Having the effect of counteracting diarrhea.