The most common and most likely answer to this crossword clue is the 6 letter word KNAVES
I've seen this crossword clue in The Daily Telegraph
K
N
A
V
E
S
Buddy explains!

The straight part
We know that "Cards" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cards = knaves.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 KNAVES mean?
KNAVES noun- A boy; especially, a boy servant.
- Any male servant; a menial.
- A tricky, deceitful fellow; a dishonest person.
- A playing card marked with the figure of a servant or soldier; a jack.