The most common and most likely answer to this crossword clue is the 3 letter word HIT
H
I
T
More clues leading to the result HIT
Buddy explains!

The straight part
We know that "Knock/successful song" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knock/successful song = hit.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 HIT mean?
HIT noun- A blow; a punch; a striking against; the collision of one body against another; the stroke that touches anything.
- Something very successful, such as a song, film, or video game, that receives widespread recognition and acclaim.
- An attack on a location, person or people.
- A collision of a projectile with the target.
- A match found by searching a computer system or search engine
- A measured visit to a web site, a request for a single file from a web server.
- An approximately correct answer in a test set.
- The complete play, when the batter reaches base without the benefit of a walk, error, or fielder’s choice.
- A dose of an illegal or addictive drug.
- A premeditated murder done for criminal or political purposes.
- A peculiarly apt expression or turn of thought; a phrase which hits the mark.
- A move that throws one of the opponent's men back to the entering point.
- A game won after the adversary has removed some of his men. It counts for less than a gammon.
- (heading, physical) To strike.
- To manage to touch (a target) in the right place.
- To switch on.
- To briefly visit.
- To encounter an obstacle or other difficulty.
- (heading) To attain, to achieve.
- To affect negatively.
- To attack.
- (heading, games) To make a play.
- To use; to connect to.
- To have sex with.
- To inhale an amount of smoke from a narcotic substance, particularly marijuana.
- Very successful.
- It.