The most common and most likely answer to this crossword clue is the 13 letter word SUPERPOSITION
I've seen this crossword clue in The Times
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More clues leading to the result SUPERPOSITION
Buddy explains!
The straight part
We know that "Act of putting over" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act of putting over = superposition.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 SUPERPOSITION mean?
SUPERPOSITION noun- The placing of one thing on top of another.
- The deposition of one stratum over another; the principle that in a series of sedimentary strata, the lower strata are the older.
- The situation in quantum mechanics where two or more quantum states are added together (superposed) to yield another valid quantum state.