1 --- on-Tees, starting point for the world's first passenger railway, 1825 (8)
9 Cohort, under-16s, for example (3,5)
10 South American capital (4)
11 Form of an element (12)
13 A court of public records (8)
15 King of England 1053-1066 (6)
16 Political grouping (4)
17 Long-handled oar mounted at the stern of a boat and moved left and right to propel the boat forward (5)
18 Founder of modern communism (4)
20 Musical timbre (6)
21 A kind of bitter stout brewed in Ireland (8)
23 Greek author of The Wasps and Lysistrata (12)
26 Ballet skirt (4)
27 Large South American snake of the boa family (8)
28 Any of various cud-chewing hoofed mammals having a stomach divided into four (occasionally three) compartments (8)

2 Having three groups of hydrocarbon radicals (8)
3 London railway terminus (7,5)
4 Colour a garment after knotting (3,3)
5 Collective security organization (abbr.) (4)
6 William ---, astronomer and discoverer of the planet Uranus (8)
7 The boundary between the rocks of the Earth's crust and the mantle (4)
8 A supplement; vestigial organ (8)
12 Asian country, capital Ashkhabad (12)
14 Neil, Canadian rock singer, songwriter and guitarist (5)
16 Card game played in casinos (8)
17 Theological college of the medieval university of Paris (8)
19 Grigory Efimovich Novykh, court favourite of the wife of Nicholas II (8)
22 An 11 across of Thorium (6)
24 Middle Eastern country (4)
25 Ice crystals forming a white deposit (4)