Cheers, dizoz - never heard of it before. I see there are alternatives, so it might take a lot of experimentation.
I'm assuming that you have to associate one letter with each clue, and then put into the clue the word the letter is 'for', and that none of this is indicated by the clue as given to the solver. The instructions seem a bit 'foggy'/badly written: I asked DH for help interpreting them and his first question was, "Does it mean there's a word in each clue made up of some of those 13 letters?". We then came up with alternative ways the instructions might work. I think instructions should be clear, otherwise what's the point?
(Felt much happier after giving up in disgust, having some key lime pie & strawberries and watching a recording of last night's 'Only Connect' and scoring highly against the contestants - yay!)
I did get one answer immediately I read it last night: 13D, I think. But then I ground to a halt. Will print out the cockney slang list from wiki & try again.
I also notice that you gave this hint as post #2. Thank you for your patience, dear dizoz! How far have you got with the actual crossword?