Cryptic crossword No 2,875

Across

1. Brown cereal crop (7)
2. Obscene porky umpire got rid of the last of the butter for a day of penance (3,6)
3. Hot ready to be emptied and eaten (5)
4. Understand about pounds and English anxieties (9)
5. Female has unknown sleep (4)
6. New galleries contain no parables (10)
7. Giant American state is a fairy (7)
8. Free air has been a hit in the UK since the 1950's (9,6)
9. Swimmer changes top coach for learner (5)
10. US pride evolves and comes to an end (5,2)
11. Time keeper had sex with student body from behind (7)
12. No Russian is natural up till now (4)
13. Sweet country having no church regular expresses heartache (10)

Down

1. Two articles on north eastern icon (6)
2. Mature with the speed of light in the Big Apple office (6)
3. Gold and calcium make a deadly mammal (4)
4. Rebel trades place in the Indian Ocean in a manner of speaking (10)
5. In the middle of a central thoroughfare (6)
6. Musical instruments made from Asian corn without nitrogen (8)
7. Dispatched guard without lines (4)
8. First decent young woman (6)
9. Pulse is fast and mostly unwell (6)
10. Wet and corrupt King (4)
11. Review survey with Republican after party (10)
12. Writers miss initial literary incentive vouchers (6)
13. Manage trade agreement and give out port (4)
14. Hide under step in English city (10)
15. Spiritual philosopher, endlessly charming (8)
16. Smooth musicians get birds (10)