Clue: Mother describing older doctor's excessively theatrical behaviour (9)
Answer: MELODRAMA
Explanation: Definition: excessively theatrical behaviour
**MAMA** ("mother") *describing* ***(older)** 
*I don't like the use of "doctor" as an anagrind in this clue. "Doctor" has to be used as a verb if it is to be used as an indicator, and so should come before the anagram fodder in the clue.