Youngster not about to complete plumbing challenge? (1-4)
I believe the answer is:
u-bend
'plumbing challenge?' is the definition.
'youngster not about to complete' is the wordplay.
'youngster not' becomes 'bu' (I am not sure about this - if you are sure you should believe this answer much more).
'about' shows that the letters should be reversed in order.
'to complete' becomes 'end' (I've seen this before).
'bu' written backwards gives 'ub'.
'ub'+'end'='U-BEND'
(Other definitions for u-bend that I've seen before include "Waste pipe gas trap" , "Air-trap in a pipe" , "Waste pipe feature" , "Waste pipe kink" , "Trap section of waste pipe" .)