Well acquainted with and in contact with (4)
I believe the answer is:
upon
'in contact with' is the definition.
(I've seen this before)
'well acquainted with and' is the wordplay.
'well acquainted with' becomes 'upo' (I can't justify this - if you can you should believe this answer much more).
'and' becomes 'n' (common abbreviation for 'and').
'upo'+'n'='UPON'
Can you help me to learn more?
(Other definitions for upon that I've seen before include "Abreast of, au fait with" , "Au courant" , "Carried - on top" , "On tope of" , "(Placed) on top of" .)