The phrase "clear testimony" means evidence or information that is easily understood, straightforward, and supports a certain fact or belief. Full definition
The group of temple complexes, archaeological sites and landscape along the Pakerisan River, reveals historic development of religious and architectural concepts which are clear testimony of the Hindu - Balinese cosmology concept of Tri Hita Karana, the God, human being and natural environment interlinked with each other. (blog.baliwww.com)
Flared wheel arches added on to the body both front and rear bear clear testimony to the wider track than on the former model. (ausmotive.com)
Yes, his appointment as a Fellow of Oriel College at Oxford gave clear testimony to his academic brilliance, a verdict of his peers that he vindicated with such scholarly publications as Lectures on Justification and The Arians of the Fourth Century. (firstthings.com)