The most common and most likely answer to this crossword clue is the 6 letter word MATING
I've seen this crossword clue in Independent
M
A
T
I
N
G
More clues leading to the result MATING
- Finishing game, ready to get engrossed in periodical
- Type of ritual practised before start of congress?
- Married giant — disastrous wedding
- Old Lib Dem leader keeps attending congress
- Congress assembling, with English pair replaced by American
- Pairing Asiatic tigers, initially protected by a Chinese dynasty
- Always checking king's having sex
- Marrying maiden with giant bust
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 = mating.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 MATING mean?
MATING verb- To put the king of an opponent into checkmate.
- (by extension) To place in a losing situation that has no escape.
- To match, fit together without space between.
- To copulate.
- To pair in order to raise offspring
- To arrange in matched pairs.
- To introduce (animals) together for the purpose of breeding.
- (of an animal) To copulate with.
- To marry; to match (a person).
- To match oneself against; to oppose as equal; to compete with.
- To fit (objects) together without space between.
- To move (a space shuttle orbiter) onto the back of an aircraft that can carry it.
- To confuse; to confound.
- Pairing of organisms for copulation.
- Sexual union; copulation.
- Fitting into or onto a corresponding part, as a matched plug and socket.