Down to half a rider's seat (3)
I believe the answer is:
sad
'down' is the definition.
(I've seen this before)
'half a rider's seat' is the wordplay.
'half a' indicates taking half.
'rider's seat' becomes 'saddle' (I've seen this before).
'saddle' halved is 'SAD'.
'to' is the link.
(Other definitions for sad that I've seen before include "Morose" , "Gloomy" , "Mournful and depressed" , "Melancholy, depressed" , "'Unhappy, sorry (3)'" .)