The most common and most likely answer to this crossword clue is the 5 letter word NOOKY
I've seen this crossword clue in The Guardian
N
O
O
K
Y
More clues leading to the result NOOKY
Buddy explains!

The straight part
We know that "Congress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case congress = nooky.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 NOOKY mean?
NOOKY noun- Sexual intercourse or other sexual activity, especially if illicit.
- A pacifier.
- Like a nook; small and secluded or cosy.