Established outlet only closed after nag departed? That's too late (6,4)
I believe the answer is:
stable door
'only closed after nag departed? that's too late' is the definition.
I can't judge whether this definition defines the answer.
'established outlet' is the wordplay.
'established' becomes 'stable' (I can't explain this - if you can you should give a lot more credence to this answer).
'outlet' becomes 'door' (I've seen this in another clue).
'stable'+'door'='STABLE DOOR'
Can you help me to learn more?
(Another definition for stable door that I've seen is " It's often locked after the horse has bolted".)