Neither is in ignorance (3)
I believe the answer is:
nor
'neither' is the definition.
(eg in the phrase 'neither do I')
'in ignorance' is the wordplay.
'in' indicates a hidden word.
'NOR' is hidden within 'ignorance'.
'is' acts as a link.
(Other definitions for nor that I've seen before include "Common conjunction, goes with neither" , "logic circuit" , "It's also not" , "Conjunction meaning 'and not'" , "Negative conjunction" .)