Crossword Genius

I want a head sacked! Preposterous (4,2,4)

Ross

I believe the answer is:

what an idea

'preposterous' is the definition.
(what a preposterous idea)

'i want a head sacked' is the wordplay.
'sacked' indicates anagramming the letters (sack can mean to plunder or destroy).
'i'+'want'+'a'+'head'='iwantahead'
'iwantahead' anagrammed gives 'WHAT AN IDEA'.

I've seen this clue in The Guardian.
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