Lord has to welcome London Transport's founder (2,4,3,5)
I believe the answer is:
go down the tubes
'founder' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'lord has to welcome london transport's' is the wordplay.
'lord' becomes 'God' (the Lord means God).
'has' becomes 'owns' ('own' can be a synonym of 'have').
'to welcome' is an insertion indicator (inserted letters are welcomed in).
'london transport's' becomes 'thetube' (I am not sure about this - if you are sure you should believe this answer much more).
'owns' enclosing 'thetube' is 'ownthetubes'.
'god'+'ownthetubes'='GO DOWN THE TUBES'
Can you help me to learn more?
(Another definition for go down the tubes that I've seen is " Fail".)