Here's the story and you'd think if I could remember all this detail I'd know the name:
A man is selected by some random process to represent all of Earth and he's being interviewed by an alien galactic empire. The powers on earth, thinking they are bound to fail the test, rig him up so that it appears he has a subspace radio communicating back to Earth in secret- but it's all a ploy to make the aliens believe we have subspace radio technology even though we've discovered there can be no such thing.
The aliens see through the whole deceit and launch missiles to destroy earth - not only are we war-like and dangerous, but deceitful as well. But at the last minute the chief interrogator has a change of heart - figuring we have a unique survival trait that might be useful in an impending problem elsewhere in the galaxy and orders the missiles destroyed - that instruction being sent by subspace radio! Really neat short story - it was in an anthology of short stories probably by the same author published c1960. Brian Aldiss?