Not even louts stay robust (5)
I believe the answer is:
lusty
'robust' is the definition.
(synonyms)
'not even louts stay' is the wordplay.
'not even' indicates alternate letters (remove even-numbered letters).
'louts'+'stay'='loutsstay'
The alternating letters of 'loutsstay' are 'LUSTY'.
(Other definitions for lusty that I've seen before include "Full of health, vigour" , "Full-blooded" , "Full of strength and energy" , "Stout" , "Brawny" .)