Crossword Genius

Knave fit jars into position (7)

Ross

I believe the answer is:

ratfink

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'knave' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

'fit jars into position' is the wordplay.
'jars' indicates anagramming the letters (the letters must be shocked or jarred).
'into' indicates putting letters inside.
'position' becomes 'rank' (rank is a kind of position).
'fit' with letters rearranged gives 'tfi'.
'tfi' placed inside 'rank' is 'RATFINK'.

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I've seen this clue in the Sydney Morning Herald.
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