16d- is act arising in long story happening from time to time (8) - ?P???M?? - epidemic?? I can see epic but not the rest if this answer is correct.
21ac- athlete - small one making an impression (8) - ????N?E? - if epidemic is right, then this answer begins with "d".
Thanks Penelope and Jack. That means my answers to these clues were wrong!!
17d- loudly criticises accommodation (8) - ?A?R????
21ac- avoid failure to score (4) - D???