### Clue: Smashing fellow deemed mad is lying in grass (10)
### Answer: SHATTERING 
 ### Explanation: Definition: Smashing
 HATTER(in full, the Mad Hatter, the fellow deemed mad in the tale, "Alice in Wonderland") **contained in**(is lying in) SING(to grass/to rat on/to inform on one's accomplice, say, especially to the authorities).