Rough hop over grassland (8)
I believe the answer is:
ball park
'rough' is the definition.
(I've seen this before)
'hop over grassland' is the wordplay.
'hop' becomes 'ball' (both can mean an event with dancing).
'over grassland' becomes 'park' (I can't justify this - if you can you should give a lot more credence to this answer).
'ball'+'park'='BALL PARK'
(Other definitions for ball park that I've seen before include "(Of a figure) approximate" , "(Of an estimate) approximate" , "Approximate range" , "Sphere of (game or) activity" , "Rough" .)