I shall not be in good health (3)
I believe the answer is:
ill
'i shall' is the definition.
(contraction of 'I will')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ill that I've seen before include "Badly diminished" , "Not in good health" , "poor" , "Sickly" , "trouble" .)