Flexible game (6)
I believe the answer is:
rubber
'flexible' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
'game' is the wordplay.
I cannot really understand how this works, but
'game' could be 'ru' (rugby union) and 'ru' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rubber that I've seen before include "Stretchy substance" , "Boring, obsessive type" , "Elastic substance from trees" , "a bit of bridge" , "Latex from trees" .)