The most common and most likely answer to this crossword clue is the 8 letter word BRAINING
I've seen this crossword clue in The Daily Telegraph
B
R
A
I
N
I
N
G
Buddy explains!

The straight part
We know that "hitting head" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hitting head = braining.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 BRAINING mean?
BRAINING verb- To dash out the brains of; to kill by smashing the skull.
- To strike (someone) on the head.
- To destroy; to put an end to.
- To conceive in the mind; to understand.