South, down in card game (4)
I believe the answer is:
snap
'card game' is the definition.
(I've seen this before)
'south down' is the wordplay.
'south' becomes 'S' (synonyms).
'down' becomes 'nap' (close in meaning in sense of hairy surface).
's'+'nap'='SNAP'
'in' acts as a link.
(Other definitions for snap that I've seen before include "Crack; bite" , "piece of cake" , "Break sharply - a simple card game" , "Break with a crack" , "Unpremeditated - bite" .)