Crossword Genius

Completely fail to take the Underground, son? (2,4,3,5)

Ross

I believe the answer is:

go down the tubes

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'completely fail' is the definition.
'go down the tubes' can be an answer for 'fail' (I've seen this before). I am not certain of the 'completely' bit.

'take the underground son?' is the wordplay.
'take' becomes 'godown' (I can't explain this - if you can you should believe this answer much more).
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'son?' becomes 's'.
'godown'+'the'+'tube'+'s'='GO DOWN THE TUBES'

'to' is the link.

Can you help me to learn more?

(Another definition for go down the tubes that I've seen is " Fail".)

I've seen this clue in The Guardian.
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