Invalid making a comeback in artificial lung (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'a comeback in artificial lung' is the wordplay.
'a comeback' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'llun' is hidden in the letters of 'artificial lung'.
'llun' reversed gives 'NULL'.
'making' is the link.
(Other definitions for null that I've seen before include "... and void, lacking any legal or binding force" , "Immaterial and void" , "Inoperative" , "Invalid - not binding" , "Without value or consequence" .)