Guard-dog’s warning about everyone beginning to picket sports ground (4,4)
I believe the answer is:
ball park
'sports ground' is the definition.
(baseball ground)
'guard-dog's warning about everyone beginning to picket' is the wordplay.
'guard-dog's warning' becomes 'bark' (dogs bark).
'about' indicates putting letters inside.
'everyone' becomes 'all' (all people).
'beginning to' indicates taking the first letters.
The first letter of 'picket' is 'p'.
'all'+'p'='allp'
'bark' placed around 'allp' is 'BALL PARK'.
(Other definitions for ball park that I've seen before include "Baseball ground; particular area" , "Sphere of (game or) activity" , "(Of an estimate) approximate" , "Approximate range" , "Scene of American game" .)