Horse briefly on mark (5)
I believe the answer is:
colon
'mark' is the definition.
(colon is a kind of mark)
'horse briefly on' is the wordplay.
'horse' becomes 'colt' (I've seen this before).
'briefly' means to remove the last letter.
'colt' with its last letter taken away is 'col'.
'col'+'on'='COLON'
(Other definitions for colon that I've seen before include "cash" , "almost the end of another" , "unit of foreign currency" , "that may cut sentence" , "Part of the intestine - punctuation mark" .)