lumen - It's the definition that isn't in Chambers, the word is there with a perfectly adequate (although different) definition.
'A' is an obscure double def, so not a very helpful clue - for a more helpful synonym try, 'marsh plants.'
'k' Turn the Greek island backwards to begin with.
A bit more on 'l' might be necessary - How you would ask a dietician to rid you of the excess of nasties you've acquired by eating too many spuds & loaves (5, 2) and reverse it [long? Yes - but funny, I liked it}.
'm' isn't really cryptic - try 'screenwriter' + the name.