The most common and most likely answer to this crossword clue is the 6 letter word POSING
I've seen this crossword clue in The Daily Telegraph
P
O
S
I
N
G
Buddy explains!
The straight part
We know that "putting on an act" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case putting on an act = posing.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 POSING mean?
POSING verb- To place in an attitude or fixed position, for the sake of effect.
- To ask; to set (a test, quiz, riddle, etc.).
- To constitute (a danger, a threat, a risk, etc.).
- (in the phrase "to pose as") To falsely impersonate (another person or occupation) primarily for the purpose of accomplishing something or reaching a goal.
- To assume or maintain a pose; to strike an attitude.
- To behave affectedly in order to attract interest or admiration.
- To interrogate; to question.
- To question with a view to puzzling; to embarrass by questioning or scrutiny; to bring to a stand.
- To ask (someone) questions; to interrogate.
- To puzzle, non-plus, or embarrass with difficult questions.
- To perplex or confuse (someone).
- The act by which something is posed.
- The act of one who poses or postures.