Thanks for all your suggestions. I'd never heard of a half-diminished seventh, but that is what it is. Also F#m6 is OK as it gives all the right notes, even if not in the right order (now I'm sounding like Eric Morecambe). I also like 'B9 rootless', never heard that before.

D#m6 can't be right as that would give you an A#, not A. Bflat dim 7 is three-quarters wrong.

Out of interest, the context is that the chord is preceded and followed by a chord of A with an E in the bass, i.e. second inversion. (The song is the McGarrigles' 'Work Song').

Naming chords is important, otherwise how do you tell the guitarist what you want, short of playing it for him?