### Clue: Suddenly stop riding? Time to get your skates on (6,2)
### Answer: FREEZE UP 
 ### Explanation: Definition: Time to get your skates on
  FREEZE(suddenly stop, as in "freeze! don't move!" or when a computer programme stops working for apparently no reason) + UP(on a horse;riding).
Defn:  A period of freezing weather (shouldn't there be a hyphen to indicate that it's a noun?).