The phrase "clear allusion" means that something is being mentioned or referred to indirectly, but it is very obvious and easy to understand. Full definition
When the writers of the New Testament hold up this aspect of his life for imitation, they make clear allusions to Isaiah 53. (religion-online.org)
«Vicente's divertimientos range from non-representational explorations of color, texture, plane, and volume to cleverly - concocted figures with clear allusions to human forms and animals. (greyartgallery.nyu.edu)
2, 30, 9); but this is the only clear allusion in his writings, and he speaks of the Christian «altar in the heavens» (4, 18, 6) in such a way as to show that he is not relying on what Hebrews has to say on the subject. (religion-online.org)