Thanks! :)
I looked at my reasonable options with only one button and that were: click, double click and hold click :D (a triple click or something like this would have been too much I would say)
But I had on my to-do list to add three little animated icon-pairs to the bottom right corner indicating the possible actions a player would have and swaping the second icon out depending on the situation (e.g. movement an arrow forward, but if in front of an enemy, that icon-pair would show the hold-icon and some sort of sword clashing), but... time and my lack of graphic skills said no :D