  Jellyfish galaxies have long tails of gas that is stripped from the disc by
ram pressure due to the motion of galaxies in the intracluster medium in galaxy
clusters. We present the first measurement of the magnetic field strength and
orientation within the disc and the (90$\,$kpc-long) $\rm H\alpha$-emitting
tail of the jellyfish galaxy JO206. The tail has a large-scale magnetic field
($>4.1\,\mu$G), a steep radio spectral index ($\alpha \sim -2.0$), indicating
an aging of the electrons propagating away from the star-forming regions, and
extremely high fractional polarisation ($>50\,$%), indicating low turbulent
motions. The magnetic field vectors are aligned with (parallel to) the
direction of the ionised-gas tail and stripping direction. High-resolution
simulations of a large, cold gas cloud that is exposed to a hot, magnetised
turbulent wind show that the high fractional polarisation and the ordered
magnetic field can be explained by accretion of draped magnetised plasma from
the hot wind that condenses onto the external layers of the tail, where it is
adiabatically compressed and sheared. The ordered magnetic field, preventing
heat and momentum exchange, may be a key factor in allowing in-situ star
formation in the tail.
