Strong vehicle covered in decay (6)
I believe the answer is:
robust
'strong' is the definition.
('robust' can be similar in meaning to 'strong')
'vehicle covered in decay' is the wordplay.
'vehicle' becomes 'bus' (I've seen this before).
'covered in' indicates putting letters inside.
'decay' becomes 'rot' (rot is a kind of decay).
'bus' put within 'rot' is 'ROBUST'.
(Other definitions for robust that I've seen before include "Physically strong" , "Turbos (anag) - fit for purpose!" , "Athletic" , "Tough!" , "Hardy and vigorous" .)