God taking part in horse race coming up (4)
I believe the answer is:
ares
'god' is the definition.
(Greek god of war)
'part in horse race coming up' is the wordplay.
'part in' says the answer is hidden in the clue.
'coming up' is a reversal indicator (in a down clue, letters go up).
'sera' is hidden in the letters of 'horse race'.
'sera' reversed gives 'ARES'.
'taking' is the link.
(Other definitions for ares that I've seen before include "Greek god of war" , "War-god" , "Olympian" , "Belligerent god" , "Land measures" .)