I'm unable to finish this crossword because of one seemingly simple clue.
49D, Edible seaweed (5), _ I _ E _
I know what you're going to say, the answer's LAVER and the I is wrong.
However the I is part of ...
51A, Definite and clear (8), E _ P _ ? _ I _
where the ? is the I in 49D because I'm convinced that 51A is EXPLICIT.