The underground is in some trouble (5)
I believe the answer is:
Here is my best explanation:
'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 "what moves many Parisians", "Underground rail system", "Parisian underground system", "Underground railway", "Transport system", "Urban railway system", "Paris underground rail system", "The underground railway system in Paris", "Paris subway" and "Parisian underground railway".)