### Clue: Certain old penniless escort on an 'orse (10)
### Answer: GUARANTEED 
 ### Explanation: Definition: Certain
A charade of GUAR[d] ('escort') minus D ('old penniless') *plus 'an'* plus [s]TEED (h'orse', with the apostrophe indicating the dropping of the first letter, not, as is usual, specifically an H).