How to define the circuit that replaces the variable capacitor in its use with the TDA7000?
TDA7000 description and internal block diagram here 

As Brian notes - D1 is a varicap - a diode with voltage variable capacitance. 

Q1 and associated circuitry provides a fixedish DC voltage across R7+P2.
This is tapped off by P2 wiper and used to DC bias (tune) the varicap. 

TDA7000 pin6 provides the other end of the varicap voltage - this is internal connected to a VCO and internal varicaps. SO it seems that the circuitry in question is part of a stable DC tuning supply.