Curt mostly follows flat horses (8)
I believe the answer is:
eventers
'horses' is the definition.
(horse taking place in eventing)
'curt mostly follows flat' is the wordplay.
'curt' becomes 'terse' ('terse' can be a synonym of 'curt').
'mostly' means to remove the last letter.
'follows' says to put letters next to each other.
'flat' becomes 'even' ('even' can be similar in meaning to 'flat'**).
'terse' with its final letter taken away is 'ters'.
'ters' after 'even' is 'EVENTERS'.
(Another definition for eventers that I've seen is " Horse riders".)