Room to put a horse's price on a card (5)
I believe the answer is:
space
'room' is the definition.
(room can mean space or an empty area)
'a horse's price on a card' is the wordplay.
'a horse's price' becomes 'sp' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'a card' becomes 'ace' (ace is a type of playing card).
'sp'+'ace'='SPACE'
'to put' acts as a link.
Can you help me to learn more?
(Other definitions for space that I've seen before include "Expanse" , "Unfilled region" , "Cosmos" , "Emptiness" , "Interstellar region" .)