Smoker on the tube (4)
I believe the answer is:
pipe
I believe this clue is a double definition.
'smoker' is the first definition.
'the tube' is the second definition.
(pipes are tubular)
'on' acts as a link.
(Other definitions for pipe that I've seen before include "Tube for conveying gas or liquid" , "Whistle" , "Wine cask; sing shrilly" , "Smoker's item" , "Transmit through a tube" .)