Partner means one of a couple, whether of the same sex or of the opposite sex (the other being a Member) who although not married to each other or civil partners are living together and treat each other as spouses.
The clue is in the final five words. And that, ladies and gentlemen is why it ought to be difficult for the Standards Commissioner to do anything other than clear David Laws. Neither he nor Jamie Lundie thought of the other, or treated the other, as a spouse.
Unfortunately it won't do him much good as the word is that David wants to quit politics altogether.
Politics really has come to a parlous state when most people will treat this eventuality with a massive shrug of the shoulders and say 'so what'. The court of public opinion has already found David Laws guilty, and in the end that's what matters nowadays. All politicians are guilty as charged, no matter how trumped up the charges might be.
And that ladies and gentlement is part of the reason why many people - me among them - want nothing more to do with national elected politics.