Usually: Worn caliper hardware or loose brake pads shifting in the bracket.
But it can also be: Worn suspension components that only load up under braking, such as ball joints or strut mounts.
Clunk when you brake
A clunk that shows up right when you first press the brake pedal is usually the brake pads themselves. Pads sit in a bracket with clips and a bit of hardware to hold them snug. Once that hardware wears down, or the clips lose their tension, the pad has a little room to move. Press the brake, the pad shifts into the caliper, and you hear a clunk. It is a cheap, harmless-sounding noise that still needs attention, because a loose pad wears unevenly and can start to squeal or grab oddly.
The other common cause is further up the chain: worn suspension parts that only load up under braking, like ball joints, control arm bushings, or strut mounts. These stay quiet most of the time because braking is one of the few moments that puts real force through them front-to-back instead of up-and-down.
How to tell the difference
Find an empty parking lot and roll slowly, then brake gently. If the clunk happens right as the pedal engages, and you can sometimes get it to happen even at a near-stop with light pressure, that points at the pads and hardware. If the clunk only shows up under firmer braking, especially combined with a bump in the road, or if you also feel a vague looseness in the steering when going over rough pavement, that points more toward suspension.
Another quick check: with the car off and parked, grab the top of a front tire and rock it in and out (not side to side). Looseness or a clunk there suggests worn suspension components rather than just the brakes.
Is it safe to keep driving
A single clunk on brake application is rarely an immediate safety problem. Your brakes are still stopping the car. But it means something has worn past where it should sit, and that only gets worse. Worn ball joints in particular are worth not ignoring for long, since they affect how the wheel stays attached to the car.
What fixing it usually involves
If it is hardware, the fix is new clips, shims, or an anti-rattle kit, sometimes done at the same time as a pad replacement. That is a straightforward job. If it turns out to be suspension, the repair depends on which part is worn, and that can range from a single bushing to a ball joint or strut mount replacement. We would put the car on the lift, check the pads and hardware first since that is the more common and cheaper answer, and only move to suspension components if the brake side checks out fine.