Initially very ill, really unwell . . . suffering this? (5)
I believe the answer is:
virus
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'initially very ill really unwell suffering' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'very ill really unwell suffering' is 'VIRUS'.
Can you help me to learn more?
(Other definitions for virus that I've seen before include "Infective particle" , "One infecting human, computer" , "Pathogenic agent" , "Microscopic entity capable of reproducing within cells" , "Bodily or computer infection" .)