Tough case of decay limiting public transport (6)
I believe the answer is:
robust
'tough' is the definition.
(thesaurus)
'decay limiting public transport' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'limiting' means one lot of letters goes inside another.
'public transport' becomes 'bus' (bus is a kind of public transport).
'rot' enclosing 'bus' is 'ROBUST'.
'case of' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for robust that I've seen before include "Vigorous and hearty" , "firm!" , "Having a strong constitution" , "Rugged and sturdy" , "Sturdy and hardy" .)