Play down south of Texas, say (10)
I believe the answer is:
understate
'play down' is the definition.
(I've seen this before)
'south of texas say' is the wordplay.
'south of' becomes 'under' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'texas say' becomes 'state' (texas is a kind of state).
'under'+'state'='UNDERSTATE'
Can you help me to learn more?
(Other definitions for understate that I've seen before include "Speak with restraint" , "Minimise the significance of (something)" , "Play down" , "Downplay" , "Represent as less important" .)