The most common and most likely answer to this crossword clue is the 4 plus 2 plus 8 letter word KING OF DIAMONDS
I've seen this crossword clue in The Daily Telegraph
K
I
N
G
O
F
D
I
A
M
O
N
D
S
More clues leading to the result KING OF DIAMONDS
Buddy explains!
The straight part
We know that "He’s a card" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case he’s a card = king of diamonds.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.