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 subsidiary indication.
'in' indicates the answer is hidden within the clue.
'metro' can be found hidden inside 'some trouble'.
'is' is the link.
(Other definitions for metro that I've seen before include "Parisian underground system", "The underground railway system in Paris", "Underground railway", "Underground rail system", "Urban railway system", "Paris underground rail system", "French underground", "Paris subway", "Parisian underground railway" and "Parisian rail system".)