The most common and most likely answer to this crossword clue is the 5 letter word THREE
I've seen this crossword clue in The Times
T
H
R
E
E
More clues leading to the result THREE
- Trinity right within you
- Fairly good hotel in a street in resort, close to pier
- We help in their shambolic call to vote
- The vet yearned to reform equestrian contest
- Its entrants are bound to stagger, cheered in old country
- Article about Religious Education Cardinal's produced
- Rugby players — 75% of them?
- Harry Potter hurt nine, which drivers sometimes can do
- Religious class absorbed by the Trinity
- Fifty percent of members bound to participate in this event?
Buddy explains!

The straight part
We know that "Fifty percent of members bound to participate in this event?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fifty percent of members bound to participate in this event? = three.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 THREE mean?
THREE noun- The digit/figure 3.
- Anything measuring three units, as length.
- A person who is three years old.
- The playing card featuring three pips.
- A numerical value after two and before four. Represented in Arabic digits as 3; this many dots (•••).
- Describing a set or group with three elements.
- A goal or other means of scoring worth three points.
- A three-point shot; a basket made from beyond the three-point line.
- A three-point landing; an airplane landing in which all three wheels touch the ground simultaneously.
- A deer or other animal whose antlers feature three points or tines.