Robust, strong (6)
I believe the answer is:
sturdy
I believe this clue is a double definition.
'robust' is the first definition.
(similar in meaning)
'strong' is the second definition.
(similar in meaning)
(Other definitions for sturdy that I've seen before include "Strong, solid" , "Strong and stout" , "Stout, brawny" , "Stalwart, stout, tough" , "Firm" .)