  We discuss a necessary nonvalence contribution in timelike exclusive
processes. Following a Schwinger-Dyson type of approach, we relate the
nonvalence contribution to an ordinary light-front wave function that has been
extensively tested in the spacelike exclusive processes. A complicate
multi-body energy denominator is exactly cancelled in summing the light-front
time-ordered amplitudes. Applying our method to $K_{\ell3}$ and $D^0\to K^-
\ell^+ \nu_l$ where a rather substantial nonvalence contribution is expected,
we find not only an improvement in comparing with the experimental data but
also a covariance(i.e. frame-indepndence) of existing light-front constituent
quark model.
