Nigella gelatinised revolting stuffing — that's valid! (5)
I believe the answer is:
legal
'valid' is the definition.
(similar in meaning)
'nigella gelatinised revolting stuffing' is the wordplay.
'stuffing' indicates the answer is hidden within the clue.
'revolting' says the letters should be written backwards.
'lagel' is hidden in the letters of 'nigella gelatinised'.
'lagel' in reverse letter order is 'LEGAL'.
'that's' is the link.
(Other definitions for legal that I've seen before include "Like some tender" , "Relating to jurisprudence" , "Above board" , "Legitimate" , "In accordance with statute" .)