Frail, weak (6)
I believe the answer is:
infirm
I believe this clue is a double definition.
'frail' is the first definition.
(similar in meaning)
'weak' is the second definition.
(synonyms)
(Other definitions for infirm that I've seen before include "Not physically strong" , "Physically weak as from old age" , "Sickly" , "Debilitated" , "Feeble from age" .)