Possibly impure advance in the sack (7)

I believe the answer is:

blended

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'possibly impure' is the definition.
I can't tell whether this definition defines the answer.

'advance in the sack' is the wordplay.
'advance' becomes 'lend' (I've seen this before).
'in' is an insertion indicator.
'the sack' becomes 'bed' ('in the sack' means 'in bed').
'lend' inserted within 'bed' is 'BLENDED'.

Can you help me to learn more?

(Another definition for blended that I've seen is " Mixed together".)

I've seen this clue in The Times.

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