LONDON (Reuters) - Britain's Marks & Spencer reported on Wednesday a 17% drop in first-half profit, dragged down by falling clothing sales, illustrating the bumpy path of its latest attempt at a turnaround after over a decade of false dawns.Shares in the 135-year-old M&S, one of the best known names in British retail, have fallen 36% over the last year and in September the group lost its place in the prestigious FTSE 100 index.But the stock was up 5.7% at 0807 GMT after it said it was planning..