U-shaped part of a drainpipe (4)
I believe the answer is:
trap
'u-shaped part of a drainpipe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trap that I've seen before include "Part (anag.)" , "two-wheeler" , "Confine" , "Light carriage; ensnare" , "Light carriage - it's sometimes sprung!" .)