The most common and most likely answer to this crossword clue is the 3 plus 2 letter word PUT ON
I've seen this crossword clue in Financial Times
P
U
T
O
N
Buddy explains!
The straight part
We know that "Attach to/stak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case attach to/stak = put on.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 PUT ON mean?
PUT ON noun- A deception, hoax, or practical joke.