The underground is in some trouble (5)
I believe the answer is:
metro
'the underground' is the definition.
('metro' can be a synonym of 'underground')
'in some trouble' is the wordplay.
'in' says the answer is hidden in the clue.
'METRO' can be found hidden inside 'some trouble'.
'is' is the link.
(Other definitions for metro that I've seen before include "The underground railway system in Paris" , "what moves many Parisians" , "Paris subway" , "Urban railway system" , "Parisian underground system" .)