The most common and most likely answer to this crossword clue is the 4 plus 1 plus 5 letter word DROP A BRICK
I've seen this crossword clue in The Daily Telegraph
D
R
O
P
A
B
R
I
C
K
More clues leading to the result DROP A BRICK
Buddy explains!

The straight part
We know that "Say something tactless/lose a real friend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case say something tactless/lose a real friend = drop a brick.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.