Primarily young mares king enters in African state event for them? (8)
I believe the answer is:
gymkhana
'them?' is the definition.
I can't judge whether this defines the answer.
'primarily young mares king enters in african state event' is the wordplay.
'primarily' says to take the initial letters.
'enters' means one lot of letters goes inside another.
'in african state event' becomes 'ghana' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letters of 'young mares king' is 'ymk'.
'ymk' placed inside 'ghana' is 'GYMKHANA'.
'for' acts as a link.
(Other definitions for gymkhana that I've seen before include "Equestrian games" , "Event for horse-riders" , "A horsey 28" , "Meet at which riders and horses display skills" , "Meeting of riders and horses to display skills" .)