Invalid, somewhat ill, unable to rise (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'somewhat ill unable to rise' is the wordplay.
'somewhat' indicates a hidden word.
'to rise' says the letters should be written in reverse.
'llun' is hidden within 'ill unable'.
'llun' backwards is 'NULL'.
(Other definitions for null that I've seen before include "Immaterial and void" , "Invalid - not binding" , "Non-existent and void" , "Legally void" , "Lacking any legally binding force" .)