Uninteresting form of horse racing (4)
I believe the answer is:
flat
I believe this is a double definition.
'uninteresting' is the first definition.
(thesaurus)
'form of horse racing' is the second definition.
(horseracing without any obstacles)
(Other definitions for flat that I've seen before include "with no relief" , "Uninteresting - piece of scenery" , "Horizontal" , "Boring - apartment" , "Level; pad" .)