Play area foundation entirely covered by black stone (8,7)

'play area' is the definition.
'baseball diamond' can be an answer for 'area' (baseball diamond is a kind of area). I'm not certain of the 'play' bit.

'foundation entirely covered by black stone' is the wordplay.
I cannot quite understand how this works, but
'foundation' could be 'base' (synonyms) and 'base' is found in the answer.
'entirely' could be 'all' (all can mean entirely or completely) and 'all' is located in the answer.
'black' could be 'b' (abbreviation as in pencil classifications) and 'b' is found within the answer.
'stone' could be 'diamond' (I've seen this before) and 'diamond' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

Can you help me to learn more?