Area on sports field that is very hard (7)
I believe the answer is:
diamond
This clue is a double definition.
'area on sports field' is the first definition.
(as in a baseball diamond)
'that is very hard' is the second definition.
(diamond is a very hard substance)
(Other definitions for diamond that I've seen before include "(Usually colourless) gemstone - card" , "rock hard" , "Suit-baseball field - gem" , "shape" , "Ace" .)