Crossword Genius

Discarded material behind in drain pipe (5,7)

Ross

I believe the answer is:

waste product

I'm a little stuck... Click here to teach me more about this clue!

'discarded material' is the definition.

'behind in drain pipe' is the wordplay.
I cannot quite understand how this works, but
'behind' could be 'pro' (to be pro something is to be behind it) and 'pro' is found within the answer.
'drain' could be 'waste' (wasting is a kind of draining) and 'waste' is found in the answer.
'pipe' could be 'duct' (I've seen this in another clue) and 'duct' is found within the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for waste product that I've seen is " substance passed through".)

I've seen this clue in The Telegraph.
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