Room to put a horse's price on a card (5)
I believe the answer is:
space
'room' is the definition.
(I've seen this before)
'horse's price on a card' is the wordplay.
'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.
'a card' becomes 'ace' (ace is a kind of card).
'sp'+'ace'='SPACE'
'to put a' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for space that I've seen before include "Expanse" , "Unfilled region" , "Cosmos" , "Emptiness" , "Interstellar region" .)