Right, I looked for that once already and didn’t write it down. Always post, that’s what I say.
xmodmap -e "keysym Meta_L = Multi_key Meta_L"
This turns the left Alt key of my keyboard into a key that’s both Multi (aka Compose) to compose accentuated characters like é à and others and still Meta for my window manager.
EDIT: Actually, there is a problem with this setup: the Alt key appears to be ‘sticky’, which means that after I used it to changed frames in ION (with Alt-W) it’s switched to //Compose// mode. Instead, I now use my right Alt key, which I have never used.
xmodmap -e "keysym Meta_R = Multi_key"