Crossword Genius

Write about hour working filtration device (7)

Ross

I believe the answer is:

nephron

'filtration device' is the definition.
(part of the kidney)

'write about hour working' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'about' says the letters should be written backwards.
'hour' becomes 'hr' ('hr' can be a synonym of 'hour').
'working' becomes 'on' (**).
'pen' reversed gives 'nep'.
'nep'+'hr'+'on'='NEPHRON'

I've seen this clue in The Times.
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