Play area is mine, after polish (7)
I believe the answer is:
sandpit
'play area' is the definition.
(I've seen this before)
'mine after polish' is the wordplay.
'mine' becomes 'pit' (as in a mining pit).
'after' means one lot of letters go next to another.
'polish' becomes 'sand' (to polish or sand down).
'pit' after 'sand' is 'SANDPIT'.
'is' acts as a link.
(Other definitions for sandpit that I've seen before include "place where youngsters can play" , "Play area for small children" , "area for kids to play in" , "Excavation for children to play in" , "Site of castle perhaps" .)