One's first address (6,6)
I believe the answer is:
maiden speech
'one's' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'first address' is the wordplay.
'first' becomes 'maiden' ('maiden' can be a synonym of 'first').
'address' becomes 'speech' ('speech' can be a synonym of 'address'**).
'maiden'+'speech'='MAIDEN SPEECH'
Can you help me to learn more?
(Other definitions for maiden speech that I've seen before include "Au fait with urban life" , "original house address" , "First ever public address" , "First words in parliament" .)