1: Horseman conveyed mineral to listeners (7)
5: Administered dose of vitamin (between 100 and 1000) in head (7)
9: Discharge space-talk (5)
10: Support caliph, ultimately; reflective quote is portentous (9)
11: Before long 1000 French to be having Sunday fowl (7)
13: Navigate by terse directions (5)
14: Outspoken one-time Houston gridiron pro and mathematician (5)
15: Outfit in which one walks when outside the ship? (9)
17: Most recent gallery works may be a result of procrastination (4,5)
19: Initially, too hewed in misrepresentation to be supple (5)
21: Making out and squealing after the top comes off? (5)
22: Head of Daimler (U.K.) brought back style to noble estate (7)
23: Stop-Go see funny type of march! (5,4)
24: Old-time Persian country radio stations? (5)
27: Final recipient of product ensured indiscreetly. (3, 4)
28: Confederate poet returned with rather less sparkle (7)