Forbear to upset one's sister (6)
I believe the answer is:
resist
'forbear' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
'upset one's sister' is the wordplay.
'sister' anagrams to 'RESIST'.
Though, I'm not clear how this is indicated.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for resist that I've seen before include "Withstand a force" , "Put up opposition to sister" , "Go against" , "Check; withstand" , "Stand up to, fight against" .)