Underground workers' organisation able to confound the odds (4)
I believe the answer is:
tube
'underground' is the definition.
(the London Underground is nicknamed the Tube)
'workers' organisation able to confound the odds' is the wordplay.
'workers' organisation' becomes 'tu' (abbreviation for trade union).
'to confound the odds' means one should take alternating letters (the odd-numbered letters are removed).
The alternate letters of 'able' are 'be'.
'tu'+'be'='TUBE'
(Other definitions for tube that I've seen before include "Cylindrical hollow conduit" , "Valve" , "Pipe; underground train" , "Transport system" , "Pack of toothpaste; type of train" .)