### Clue: Expert artist keeping a model-type in proportion (3,4)
### Answer: PRO RATA 
 ### Explanation: Definition: in proportion
 PRO(short for "professional", an expert in a certain field) + RA(abbrev. for Royal Academician, a member of the Royal Academy of Arts;an artist) **containing**(keeping) [A + T(model-type of an early Ford motorcar).