Rough measurement of sports field (8)
I believe the answer is:
ballpark
'rough' is the definition.
(as in a ballpark figure)
'measurement of sports field' is the wordplay.
'measurement of sports' becomes 'ball' (I can't justify this - if you can you should give a lot more credence to this answer).
'field' becomes 'park'.
'ball'+'park'='BALLPARK'
Can you help me to learn more?
(Other definitions for ballpark that I've seen before include "place for pitchers" , "(Of a figure) approximate" , "Approximate range" , "rough area" , "sports arena" .)