  The asymmetry in the angular distribution of Drell--Yan dilepton pairs
generated in collisions where just one nucleon is transversely polarised has
been examined in the literature with a variety of results, mainly differing by
factors of two. We re-evaluate the asymmetry via twist-3 contributions in
collinear factorisation. In order to allow complete and in-depth comparison
with existing calculations, we supply all calculational details.
