British excel in equestrianism, confined here once? (9)
I believe the answer is:
bridewell
'confined here once?' is the definition.
(bridewell could historically mean a prison)
'british excel in equestrianism' is the wordplay.
I cannot really see how this works, but
'british' could be 'br' (abbreviation for British) and 'br' is located in the answer.
This explanation may well be incorrect...
(Other definitions for bridewell that I've seen before include "old prison" , "stir once" , "gaol" , "Can, perhaps" , "Former prison" .)