Completely fail to take the Underground, son? (2,4,3,5)
I believe the answer is:
go down the tubes
'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".)