One defending a game that has substance (9)
I believe the answer is:
apologist
'one defending' is the definition.
(I've seen this before)
'a game that has substance' is the wordplay.
'game' becomes 'polo' (polo is a kind of game).
'that has substance' becomes 'gist' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'polo'+'gist'='APOLOGIST'
Can you help me to learn more?
(Other definitions for apologist that I've seen before include "he puts up a defence" , "Champion" , "Justifier" , "Person who argues to defend or justify some policy" , "One defending the controversial" .)