Said to be acquainted with every last listener by the first of January (3,4)
I believe the answer is:
new year
'said to be acquainted' is the definition.
I can't tell whether this definition defines the answer.
'every last listener by the first of january' is the wordplay.
'last' suggests the final letters.
'listener' becomes 'ear' (body part which hears or listens).
'by' is a charade indicator (letters next to each other).
'the first of january' becomes 'new' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'every' is 'y'.
'y'+'ear'='year'
'year' after 'new' is 'NEW YEAR'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for new year that I've seen before include "Time for resolutions?" , "eg 2002" , "Near yew (anag)" , "Its eve is Hogmanay" , "Annual starting point" .)