  The location of the supergiant (sg)B[e] star Wd1-9 within the coeval
starburst cluster Westerlund 1 allows for its placement in a detailed post-Main
Sequence evolutionary scheme and hence we have utilised a comprehensive
multiwavelength dataset to determine its physical properties and relation to
other sgB[e] stars and the evolved stellar population of Wd1. Wd1-9 is found to
exhibit the rich optical emission line spectrum that is characteristic of
sgB[e] stars, while dust mass, composition and disc geometry determined from
mid-IR data resemble those of other well known examples such as R126. Extreme
historical and ongoing mass loss is inferred from radio observations, while the
X-ray properties of Wd1-9 imply the presence of high temperature plasma within
the system and are directly comparable to a number of confirmed short-period
colliding wind binaries within Wd1. The most complete explanation for the
observational properties of Wd1-9 is that it is a massive interacting binary
currently undergoing, or recently exited from, rapid Roche-lobe overflow,
supporting the hypothesis that binarity mediates the formation of (a subset of)
sgB[e] stars. The mass loss rate of Wd1-9 is consistent with such an assertion,
while viable progenitor and descendent systems are present within Wd1 and other
examples of sgB[e] binaries have previously been found. Moreover, the rarity of
sgB[e] stars - only two examples are identified from a census of ~68 young
massive Galactic clusters and associations containing ~600 post-Main Sequence
stars - is explicable given the rapidity (~10^4yr) expected for this phase of
massive binary evolution.
