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) $rm Halpha$-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.
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