  We suggest a constrained instanton (CI) solution in the physical QCD vacuum
which is described by large-scale vacuum field fluctuations. This solution
decays exponentially at large distances. It is stable only if the interaction
of the instanton with the background vacuum field is small and additional
constraints are introduced. The CI solution is explicitly constructed in the
ansatz form, and the two-point vacuum correlator of gluon field strengths is
calculated in the framework of the effective instanton vacuum model. At small
distances the results are qualitatively similar to the single instanton (SI)
case, in particular, the form factor $D_{1}$ is small, which is in agreement
with the lattice calculations.
