Clandestine humint operational techniques is a fancy way of talking about how spies gather secret information by going undercover and being sneaky.
Imagine that you and a friend are playing a game of spy. You need to gather information about what another group of kids are doing without them noticing. You might try some of these techniques:
1. Pretending to be someone else: You might put on a disguise like a wig and glasses or dress up in different clothes so that people won't recognize you as yourself. This way, you can go to the other group and pretend to be a new kid or a visitor, and ask them questions they might not normally answer.
2. Eavesdropping: You might sneak close to the other group and listen to their conversations. You can hear what they're talking about, and maybe even pick up some secret information.
3. Collecting clues: You might look around for things they've left behind, like notes or maps. If you can find these things, you might be able to figure out what they're up to.
Real life spies use these same sorts of techniques, but with much more training and skill. They go to other countries or work in other organizations, pretending to be someone they're not. They may listen in on phone calls or secretly record conversations. They might even try to recruit other people to become spies for their own side. All of these techniques are designed to get secret information without being detected.