I know what you mean about the plural, bobbinwales - it worries me too.
The clue might be pluralised and Darh has simply missed off the 's' but if it really is singular then Darh has possibly put in the wrong answer, because there's an archaic spelling of the singular naevus - NAEVE - and if the 'i' in BiMA comes from that word then it could be an 'e' (and BeMA is the usual spelling - it's in Chambers under that spelling!).
It's a pity Darh didn't come back to the thread.