But inclusion can be of an element the clue is defining, and "an" is a good enough definition for "a" I'd say.
I'd also say "one clue, two answers" would make a pretty tough WG challenge.
And speaking of tough, here's a clue from a recent Harper's that I struggled with for a long time, but in the end I think it's fair. See how you do...
The onset of Ethel Merman's first musical sound (7)
Cheers,
Mark