The most common and most likely answer to this crossword clue is the 14 letter word KNEE-TREMBLERS
I've seen this crossword clue in Financial Times
K
N
E
E
-
T
R
E
M
B
L
E
R
S
Buddy explains!

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-TREMBLERS mean?
KNEE-TREMBLERS 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.