Expressed disapproval over and over in the sack (5)

I believe the answer is:

booed

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'expressed disapproval' is the definition.
(to boo is to express disapproval)

'over and over in the sack' is the wordplay.
'over and over' becomes 'oo' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'the sack' becomes 'bed' ('in the sack' means 'in bed').
'oo' put into 'bed' is 'BOOED'.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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