17a. You just would not believe it - the Head of Clinic in such infected surroundings! (7) _ _ E _ T _ C. Creatic? Aseptic?
17d. Spurned Baker's contributions, getting upset before lying there. (7) _ _ U _ T _ _. Caustic? 1st letter of 17a must be 1st letter of 17d.
22d. Cheap and nasty paint doesn't feel quite dry yet. (5) T _ O _ _. I'm not certain about the 'o'.
26a. At such times, it sounds like one is in a dreamlike state, making one weak we hear. (4) 1st letter must be last letter of 17d and 3rd letter must be last letter of 22d.