Strong old boy in decline (6)
I believe the answer is:
robust
'strong' is the definition.
(similar in meaning)
'old boy in decline' is the wordplay.
'old boy' becomes 'bus' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'decline' becomes 'rot' (I've seen this before).
'bus' placed within 'rot' is 'ROBUST'.
Can you help me to learn more?
(Other definitions for robust that I've seen before include "Study and durable" , "Stout" , "Rugged and sturdy" , "Having a strong constitution" , "Physically strong" .)