Crossword Genius

Type of irrigation: 99? (7)

Ross

I believe the answer is:

colonic

'type of irrigation' is the definition.
(colonic irrigation cleans the colon)

'99?' is the wordplay.
I cannot really see how this works, but
'99?' could be 'ic' (Roman numerals) and 'ic' is found within the answer.
The remaining letters 'colon' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

(Another definition for colonic that I've seen is " Type of irrigation".)

I've seen this clue in The Guardian.
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