Billion invested in dull equestrian centre (6)
I believe the answer is:
stable
'equestrian centre' is the definition.
'billion invested in dull' is the wordplay.
'billion' becomes 'b'.
'invested in' is an insertion indicator.
'dull' becomes 'stale'.
'b' placed into 'stale' is 'STABLE'.
(Other definitions for stable that I've seen before include "container for punch, for instance" , "Place of birth" , "String may be kept in this" , "Steady place for a horse?" , "Horse barn" .)