### Clue: Lion in Narnia, disheartened after rejection, creates massive agency (4)
### Answer: NASA 
 ### Explanation: Definition: massive agency
 **Reversal of**(… after rejection) "Aslan"(the Lion in the Narnia Chronicles) **minus its inner letters**(disheartened).
Answer: Abbrev. for the US National Aeronautics and Space Administration.