The most common and most likely answer to this crossword clue is the 9 letter word HARA-KIRI
I've seen this crossword clue in The Times
H
A
R
A
-
K
I
R
I
Buddy explains!
The straight part
We know that "Honourable act" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case honourable act = hara-kiri.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 HARA-KIRI mean?
HARA-KIRI noun- Ceremonial suicide by disembowelment, as by slicing open the abdomen with a dagger or knife: formerly practised in Japan by the Samurai when disgraced or sentenced to death.
- Suicide or any suicidal action.
- (by extension) An act against one's own interests.