Vouches for the truth of

The clue ‘Vouches for the truth of‘ means that the answer is a verb which indicates that someone is declaring or asserting something to be true. The answer to this crossword question is ‘SWEARSTO‘. To ‘swear to’ something means to make a solemn declaration or promise that what you are saying is true, usually under oath. Therefore, ‘swear to’ fits perfectly as the answer to the crossword question as it indicates that someone is vouching for the truth of something by declaring it to be true under oath or with great confidence.