### Clue: Physicist's eccentric old relative grasps mass (7)
### Answer: FEYNMAN 
 ### Explanation: Definition: Physicist
 FEY(eccentric;whimsical) + NAN(child's word for a grandmother, an old relative) **containing**(grasps) M(symbol for "mass" in physics).
Answer: Richard, US theoretical physicist known for his work in 17across 1down, Nobel Laureate, and interesting character. His technical and non-technical lectures and books by him and about him are highly recommended reading.