Certain old penniless escort on an ’orse (10)
I believe the answer is:
guaranteed
'certain' is the definition.
(guaranteed can mean certain or definite)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for guaranteed that I've seen before include "Supported" , "Warranted" , "Ensured" , "swore" , "protected" .)