Thanks for the explanation of 15, but I think it's a pretty poor clue! Is the surface then supposed to read that if you pour out (and drink) most of the whiskey, you will do the answer later?? Very clumsy, if that is the case, but maybe I am missing something else?
Thanks PS for the explanation of the 'an' in 13. I sometimes find it hard to know what criteria is used to indicate just the first letter of a word, when the clue doesn't also use an indicator like 'primarily' or 'firstly'. Or is part of the Nato alphabet, as you say above - and I'm pretty sure that negroni is not part of it!
I've now solved10a - bit of a doh moment!