Hi Hubert,
thank you for your question. It would be much more easier to answer if you can share project files which shows the example of such JSON string and function block settings.
If you want to access JSON array, just use "[x]" notation, i.e. weather[0].main (means field "main" in 1st item of "weather" array) - see "0302-04 OpenWeatherMap API" example.
Hope it helps.
Regards, Tomas