  We describe the partial breaking of $N=1 D=10$ supersymmetry down to $(1,0)
d=6$ supersymmetry within the non-linear realization approach. The basic
Goldstone superfield associated with this breaking is shown to be the $(1,0)
d=6$ hypermultiplet superfield $q^{ia}$ subjected to a non-linear
generalization of the standard hypermultiplet superfield constraint. The
dynamical equations implied by this constraint are identified as the manifestly
worldvolume supersymmetric equations of the Type I super 5-brane in D=10. We
give arguments in favour of existence of an appropriate brane extension of
off-shell hypermultiplet actions in harmonic superspace. Some related problems,
in particular, the issue of utilizing other $(1,0) d=6$ supermultiplets as
Goldstone ones, are shortly discussed.
