Sturdy vehicle in decay (6)
I believe the answer is:
robust
'sturdy' is the definition.
('robust' can be similar in meaning to 'sturdy')
'vehicle in decay' is the wordplay.
'vehicle' becomes 'bus' (I've seen this before).
'in' means one lot of letters goes inside another.
'decay' becomes 'rot' (rot is a kind of decay).
'bus' put inside 'rot' is 'ROBUST'.
(Other definitions for robust that I've seen before include "Athletic" , "Down-to-earth" , "Having a strong constitution" , "Vigorous and hearty" , "Sturdy, resilient" .)