1: Boxer Max returned, holding heroin, for treatment (5)
4: Aquatic plant providing a variety of materials (5,4)
8: Perfect counter-spy equipment (3)
9: Feeling threatened, suggested concealing identity (11)
11: Strap, though short, has hanger's heart (5)
12: Frost to cover the cake (3)
13: Chain some; cuff objectors (3)
14: Remove a couple of statues before writing on some tombstones (5)
15: Contrarily tough, arriving cool (9)
16: Healer making solemn statement about upcoming writers (9)
19: Headless rodent left by bird (5)
21: One final, huge Greek goddess (3)
22: Freshen the atmosphere (3)
23: Object of thinking without pen (5)
24: Person with fixed notions is pretty sore, upset about the introduction of Ebonics (11)
26: Fool to join recital (3)
27: Two affirmations in broadcast SALT agreement? (3,3,3)
28: Rushes for middle of glossed lip (5)