One bus broke down, said to return greeting in Mexico (6,4)
I believe the answer is:
buenos dias
'return greeting in mexico' is the definition.
The definition suggests a singular noun which matches the answer.
'one bus broke down said' is the wordplay.
'broke' indicates an anagram.
'down said' becomes 'dias' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one'+'bus'='onebus'
'onebus' is an anagram of 'buenos'.
'buenos'+'dias'='BUENOS DIAS'
'to' is the link.
Can you help me to learn more?
(Another definition for buenos dias that I've seen is " Good day (Spanish)".)