### Clue: Note of thirteenth letter in riddle cracked by Britain (9)
### Answer: SEMIBREVE 
 ### Explanation: Definition: Note
 [ EM(M, the thirteenth letter of the English alphabet) **contained in**(in) SIEVE(a riddle;a utensil riddled with holes used to separate coarse from finer material) ] **containing**(cracked by) BR(abbrev. for Britain).
Defn: A whole note representing 4 beats in 4/4 time.