Ev 1472

[WhiffWhaff]

40a Sister astride bad horse for riding in Mainz (7)

A letter must be changed in the clue. I think it could be ‘saddled’ or ‘saddler’ or perhaps nothing to do with saddles at all!

[Lie-in King]

Saddler - 's(iste)r' (astride; round) 'addle' (bad)

Change 'z' in Mainz to 'e' for Maine

https://en.wikipedia.org/wiki/American_Saddlebred#19th_century

^Para 2

[WhiffWhaff]

Thank you, Lie-in King. That explains it.