They clean behind empty drainpipe in segments (6)

I believe the answer is:

bidets

'they clean behind' is the definition.
(a bidet cleans your behind)

'empty drainpipe in segments' is the wordplay.
'empty' says to hollow out the word (remove centre letters).
'in' indicates putting letters inside.
'segments' becomes 'bits'.
'drainpipe' with its centre removed is 'de'.
'de' inserted inside 'bits' is 'BIDETS'.

(Another definition for bidets that I've seen is " washing facilities".)

I've seen this clue in The Guardian.

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