The most common and most likely answer to this crossword clue is the 7 plus 7 letter word KICKING ONESELF
I've seen this crossword clue in The Guardian
K
I
C
K
I
N
G
O
N
E
S
E
L
F
Buddy explains!
The straight part
We know that "Showing regret/an act of self-harm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing regret/an act of self-harm = kicking oneself.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.