Down's up, and hearts race (4)
I believe the answer is:
dash
'race' is the definition.
(dashing is a kind of racing)
'down's up and hearts' is the wordplay.
'down' becomes 'sad' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'and' says to put letters next to each other.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'sad' in reverse letter order is 'das'.
'das'+'h'='DASH'
(Other definitions for dash that I've seen before include "Hurry; destroy (hopes)" , "hasty journey" , "Run; lan" , "Hyphen - bother!" , "gratuity" .)