The most common and most likely answer to this crossword clue is the 12 letter word COUNTERPUNCH
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "Knock back" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knock back = counterpunch.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.
Crossword Answer definitions
COUNTERPUNCH noun- A punch delivered in response to a previous punch by somebody else, such as an opponent in a boxing match.
- A punch used in the cutting of other punches, often used to create the negative space in or around a glyph.
- To deliver a punch designed to exploit an opponent's momentary defensive weakness caused by a punch thrown by the opponent.
- To deliver a competitive response to an opponent designed to exploit a weakness created by the opponent's offensive efforts.