Office item used to be involved in bet (5,3)

I believe the answer is:

waste bin

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'office item' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'used to be involved in bet' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'involved' is an anagram indicator (involved can mean confusing or complex).
'in'+'bet'='inbet'
'inbet' anagrammed gives 'tebin'.
'was'+'tebin'='WASTE-BIN'

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(Other definitions for waste bin that I've seen before include "Where rubbish is" , "refuse to go in it" , "refuse collector" , "garbage can?" .)

I've seen this clue in The Guardian.

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