I believe it is 'M' followed by a word meaning 'out and out'.
Curiously when I looked up 'drone' in Chambers it didn't give the 'm' word, but when I looked up the 'm' it gave 'drone' as an alternative. But only in the app, not in the book! But if you look up 'out-and-out' in either Chambers it gives the correct 5 letter word.