Healthy transport in decay (6)
I believe the answer is:
robust
'healthy' is the definition.
(associated in meaning)
'transport in decay' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'in' indicates putting letters inside.
'decay' becomes 'rot' (rot can mean to decay).
'bus' going within 'rot' is 'ROBUST'.
(Other definitions for robust that I've seen before include "'Hearty, healthy (6)'" , "firm!" , "No-nonsense" , "Turbos (anag) - fit for purpose!" , "of sturdy construction" .)