Regret sound of horn first? Quite right (3,4)
I believe the answer is:
too true
'quite right' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'regret sound of horn first?' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'sound of horn' becomes 'toot'.
'first?' means one lot of letters go next to another.
'rue' after 'toot' is 'TOO TRUE'.
Can you help me to learn more?
(Other definitions for too true that I've seen before include "You can say that again!" , "Alas you're right" , "You're absolutely right" .)