  Nuclear effects in light nuclei due to the presence of spin-one isosinglet
6-quark clusters are investigated. The quark distributions of 6-quark clusters
are obtained by using a perturbative QCD (pQCD) based framework, which allows
us to get a good description of the ratio of the deuteron structure function to
the free nucleon structure function. Nuclear effects on the extraction of the
neutron structure functions $F_2^n$ and $g_1^n$ are estimated. We find that the
effect on the extracted spin-dependent neutron structure function is very
different from that on the spin-independent neutron structure function. The
effect enhances the Bjorken sum by about 10%, whereas its correction to the
Gottfried sum is rather small. The formalism for calculating nuclear effects is
further used to evaluate the spin-dependent structure function of the $^3$He
nucleus and a good self-consistent check is obtained.
