  The problem of time is one of the most relevant open issues in canonical
quantum gravity. Although there is a huge literature about this topic, a
commonly accepted solution has not been found yet. Here, we focus on the
semiclassical approach to the problem of time, that has the main goal of
reproducing quantum field theory on a fixed WKB background accounting also for
quantum gravity corrections. We analyze the different choices of the expansion
parameter and discuss the problems arising in previous proposals, where a
non-unitary evolution emerges as an effect of quantum gravity corrections. In
this work we develop a new approach to solve this problem by performing the WKB
expansion with the introduction of the so-called kinematical action as a clock
for quantum matter, that allows to recover a unitary dynamics.
