The most common and most likely answer to this crossword clue is the 4 plus 8 letter word KNEE TREMBLER
I've seen this crossword clue in Independent
K
N
E
E
T
R
E
M
B
L
E
R
Buddy explains!
The straight part
We know that "Encounter between upright individuals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case encounter between upright individuals = knee trembler.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 KNEE TREMBLER mean?
KNEE TREMBLER noun- A casual outdoor sexual encounter (commonly with a prostitute), usually in an alleyway or against a wall, with the man standing and the woman's knees raised.