This will be dark horse ousting leader (5)
I believe the answer is:
night
'this will be dark' is the definition.
(it's dark at night-time)
'horse ousting leader' is the wordplay.
'horse' becomes 'knight' (synonyms).
'ousting leader' means to remove the first letter (to oust is to remove).
'knight' with its initial letter taken off is 'NIGHT'.
(Other definitions for night that I've seen before include "Dark hours" , "For most, the time for sleep" , "See 8" , "Sunset to sunrise" , "Tennyson's "black bat"" .)