Cryptic crossword No 3,617

Across

1. Providing instruction through broken gate, admitting children in 1st, 2nd, 3rd and 8th place (8)
2. Witness alumnus as source of information (8)
3. Marine flora seeds each injected with Tungsten (8)
4. Go up, in case sappers trapped (8)
5. Second-rate but proper clever! (6)
6. Measure in shadow for protection (8)
7. Occasionally Kurt's up to mischief doing delivery (6)
8. Classic comedy subject to preference (2,3,4,2)
9. A long way on average, taking a short time (6)
10. Same mild eccentric facing difficult choices (8)
11. Number 10 returned in comeback of Yang's partner (6)
12. Reformed gambler (6)
13. Bulk of rodent eating with apparent hesitation (6)

Down

1. Claims spaces after deer escaped (7)
2. Rests uneasily about island refuse (6)
3. Is America holding them, confiscating drugs? Spit! (7)
4. Marshals draw back in retreating groups (8)
5. Alternatively, give a hard time (6)
6. Aussie summer treat for kids to lick with company - be messy! (3,5)
7. Lost for words after win (8)
8. Systems compromising national lists (13)
9. Sensible in charge of wandering (7)
10. Embedded in immediate locality, not vacant (8)
11. Six footers in groups (7)
12. Holding up, knees nudged secretly (6)
13. Approval of way despatched (6)