### Clue: Little Boy's younger and nastier cousin admitting personal problem to hospital doctor (1-4)
### Answer: H-BOMB 
 ### Explanation: Definition: Little Boy's younger and nastier cousin
An envelope ('admitting') of BO ('personal problem') in H ('hospital') plus MB (Medecinae Baccalaureus, 'doctor'). Little Boy was the A-bomb dropped on Hiroshima.