Swiftly (though navigable) flows the stream (4)
YA-E, and still I can't get there - haven't a clue.
Last one. Any help appreciated to end the torture....
Slander and briefly myths brought up as an afterthought (8)
_ D _ E _ D _ M
...just has to be ADDENDUM. Ok, I can see mud backwards at the end, but can you explain Nedda/Adden to me, please?...
17,19 That piece on which control's tight - that's boring! (3,3) T _ E / B _ T I want 'THE BIT' on the basis of a drill bit (boring) and the bit in a horse's mouth (control). My brain's gone. Someone...