Good open ground, initially, for retriever (3,3)
I believe the answer is:
gun dog
'retriever' is the definition.
(I've seen this before)
'good open ground initially' is the wordplay.
'good' becomes 'g' (abbreviation).
'open' becomes 'undo' (eg opening a bottle).
'initially' indicates taking the first letters.
The initial letter of 'ground' is 'g'.
'g'+'undo'+'g'='GUN-DOG'
'for' is the link.
(Other definitions for gun dog that I've seen before include "eg Pointer" , "21a 24 [GAMEKEEPER] 's hound" , "Game retriever" , "'Canine for a shooter (3,3)'" , "Hunter's friend" .)