The most common and most likely answer to this crossword clue is the 5 plus 2 letter word BOOZE UP
I've seen this crossword clue in Independent
B
O
O
Z
E
U
P
More clues leading to the result BOOZE UP
Buddy explains!

The straight part
We know that "session" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case session = booze up.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BOOZE UP mean?
BOOZE UP noun- A session of heavy drinking.