Hey 
I am new to the game, I am doing my first game so discover a bit the features and how the game works before doing a new and better game. However, I was wondering if a worker is assigned to a job, but if he can’t do his work for some reasons (lack of ressources, or winter for the pickers and so on), do we have to remove him from his job so that it will do something else like gathering wood or does he do it by itself without removing it from his work?
Thank you in advance for your help
Welcome to the forum and to the game. 
You would have to unassign that worker. You don’t have to remove them just because they can’t work for whatever reasons; it’s up to you whether you micro-manage like that. I know a lot of players do this, but personally I can’t see the point of shuffling villagers around like that since I’m never that short of workers. Try both and see which works best for your playstyle.
“Clear” seems to work well enough when you need extra workers in the wild lands outside the city.