Invalid, flipping unwell, unhappy to some extent (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'flipping unwell unhappy to some extent' is the wordplay.
'flipping' says the letters should be written in reverse.
'to some extent' says the answer is hidden in the clue.
'llun' is hidden within 'unwell unhappy'.
'llun' reversed gives 'NULL'.
(Other definitions for null that I've seen before include "... and void, lacking any legal or binding force" , "Immaterial and void" , "Without force, validity" , "Of no effect, value" , "Lacking any legally binding force" .)