What's the usage of a "Cavity resonator" in a power supply circuit of a CD player?
There is no 'cavity resonator'. The markings indicate the use of PS401 &amp; a diode in the series regulator circuit for UK only. For other markets PS401 and the diode are bypassed. Of interest in this circuit is the diode-connected-transistor Q453.

PS401 appears to be a jumper, removing which would disconnect the negative terminal of the external 9V supply jack from ground and make it redundant.
In other words, removing the jumper would allow only battery operation of the CD player.