  Giant clumps on ~kpc scales and with masses of 10^8-10^9 Msol are ubiquitous
in observed high-redshift disc galaxies. Recent simulations and observations
with high spatial resolution indicate the existence of substructure within
these clumps. We perform high-resolution simulations of a massive galaxy to
study the substructure formation within the framework of gravitational disc
instability. We focus on an isolated and pure gas disk with an isothermal
equation of state with T=10^4 K that allows capturing the effects of
self-gravity and hydrodynamics robustly. The main mass of the galaxy resides in
rotationally supported clumps which grow by merging to a maximum clump mass of
10^8 Msol with diameter ~120 pc for the dense gas. They group to clump clusters
(CCs) within relatively short times (<< 50 Myr), which are present over the
whole simulation time. We identify several mass and size scales on which the
clusters appear as single objects at the corresponding observational resolution
between ~10^8 - 10^9 Msol. Most of the clusters emerge as dense groups and for
larger beams they are more likely to be open structures represented by a single
object. In the high resolution runs higher densities can be reached, and the
initial structures can collapse further and fragment to many clumps smaller
than the initial Toomre length. In our low resolution runs, the clumps directly
form on larger scales 0.3-1 kpc with 10^8-10^9 Msol. Here, the artificial
pressure floor which is typically used to prevent spurious fragmentation
strongly influences the initial formation of clumps and their properties at
very low densities.
