Ish think of a common expression (not an actual proverb afaik) that is commonly used to complement someone who is slow in losing their temper... I think that's as far as I can go without it being an out and out spoiler!
Is the definition of 15a the first word. There is an obvious answer from my crossers but I can't make it parse and now I can't get it out of my head and think of an alternative.