“Will do!”

The word “ONIT” is a slang expression that means “on it” or “ready to go/do.” In the context of the crossword puzzle question, “Will do!” is the clue that leads to the answer “ONIT.” This suggests that the phrase “will do” is interchangeable with “on it” or “ready to do something.” Therefore, the answer to the question is that “ONIT” is a phrase that can be used to communicate that you are prepared and willing to carry out a task.