The most common and most likely answer to this crossword clue is the 4 letter word HEFT
I've seen this crossword clue in Financial Times
H
E
F
T
Buddy explains!

The straight part
We know that "A number of sheets fastened together" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a number of sheets fastened together = heft.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 HEFT mean?
HEFT verb- To lift with difficulty; to raise with some effort; to lift (a heavy thing).
- To throw, cast.
- To rise and fall.
- To utter with effort.
- To pull up with a rope or cable.
- To lift (generally); to raise, or cause to move upwards (particularly in ships or vehicles) or forwards.
- To be thrown up or raised; to rise upward, as a tower or mound.
- To displace (a vein, stratum).
- To cause to swell or rise, especially in repeated exertions.
- To move in a certain direction or into a certain position or situation.
- To retch, to make an effort to vomit; to vomit.
- To make an effort to raise, throw, or move anything; to strain to do something difficult.
- To rob; to steal from; to plunder.
- Weight.
- Heaviness, the feel of weight.
- The act or effort of heaving; violent strain or exertion.
- The greater part or bulk of anything.
- To lift up; especially, to lift something heavy.
- To test the weight of something by lifting it.
- A piece of mountain pasture to which a farm animal has become hefted (accustomed).
- An animal that has become hefted thus.
- (West of Ireland) Poor condition in sheep caused by mineral deficiency.
- (Northern England and Scotland) To make (a farm animal, especially a flock of sheep) accustomed and attached to an area of mountain pasture.
- A number of sheets of paper fastened together, as for a notebook.
- A part of a serial publication.