Their game is a silly one (4)
I believe the answer is:
mugs
'their game' is the definition.
I can't tell whether this defines the answer.
'silly one' is the wordplay.
'silly' becomes 'mug' (both can mean a foolish person).
'one' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'mug'+'s'='MUGS'
'is a' is the link.
Can you help me to learn more?
(Other definitions for mugs that I've seen before include "'Cups, faces and fall guys (4)'" , "Robs with the threat of violence" , "Gullible people" , "Attacks" , "Cups without saucers" .)