The most common and most likely answer to this crossword clue is the 6 letter word BRAVO!
I've seen this crossword clue in Independent
B
R
A
V
O
!
Buddy explains!

The straight part
We know that "Good for you!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good for you! = bravo!.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 BRAVO! mean?
BRAVO! noun- A hired soldier; an assassin; a desperado.
- A shout of "bravo!"
- The letter B in the ICAO spelling alphabet.
- To cheer or applaud, especially by saying bravo!
- Used to express acclaim, especially to a performer.