Mary rides round man's address (2,4,3)
I believe the answer is:
my dear sir
'man's address' is the definition.
(respectful term of address for a man)
'mary rides round' is the wordplay.
'round' indicates an anagram.
'mary'+'rides'='maryrides'
'maryrides' with letters rearranged gives 'MY DEAR SIR'.
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