Clues across 3 Not to be able to find something. 5 Definitely, no doubt, surely. 6 Something that you wear to hide yourself or change the way you look.

Based on the clues provided, here are the answers:

1. Clue: "Not to be able to find something." (3 letters)
   Answer: "Lose"

2. Clue: "Definitely, no doubt, surely." (5 letters)
   Answer: "Certa" (though this is more commonly "Sure" for 4 letters; "There" doesn't fit either)

3. Clue: "Something that you wear to hide yourself or change the way you look." (6 letters)
   Answer: "Masker" (for something like a mask)

So you could check "mask" and "sure" for the respective hints. If you're looking for something more specific or clarifications, feel free to ask!