Frail for a few days, say (4)
I believe the answer is:
weak
'frail' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for weak that I've seen before include "Spent" , "Faint; frail" , "lacking intensity?" , "Helpless" , "Lacking strength" .)