  The full mean-field solution of spin glass models with a continuous
order-parameter function is not directly available and approximate schemes must
be used to assess its properties. The averaged physical quantities are to be
represented via the replica trick and the limit to zero number of replicas is
to be performed for each of them. To avoid this we introduce a perturbation
expansion for a mean-field free-energy functional with a continuous
order-parameter function without the need to refer to the replica trick. The
expansion can be used to calculate all physical quantities in all mean-field
spin-glass models and at all temperatures, including zero temperature. The
small expansion parameter is a difference between the continuous
order-parameter function and the corresponding order parameter from the
solution with one level of replica-symmetry breaking. The first correction
beyond the approximation with one level of replica-symmetry breaking is
explicitly evaluated in the glassy phase of the Sherrington-Kirkpatrick model.
