Peterm - 10a is word for "fantastic" outside a 9
21d (not 21a, sorry) see my previous post. I think that if the solution is a 9, as in 17d, then a 9 is not required in the wordplay, nor is a definition given.
However, in the case of 21d, the undefined solution which I think is a 9, there seem to be two alternative cryptic clues.
a) one + hung drawn + quartered (as in provided with a quarter)
b) anag of raids with added 9
The first of these, moreover, looks like a definition for another solution that would fit, but require two 9's. I reckon Boatman is being too clever by half.