### Clue: Mares would triumph here, in sleep so monstrous (5)
### Answer: EPSOM 
 ### Explanation: Definition: Mares would triumph here
  **Hidden in**(in) sleEP SO Monstrous
Defn:  The racecourse, Epsom Downs, in Surrey, England where the annual Derby is held.  I assume mares have their fair share of triumphs at Epsom.