Square up or calm down (6)
I believe the answer is:
settle
This clue is a double definition.
'square up' is the first definition.
(synonyms)
'calm down' is the second definition.
(thesaurus)
'or' is the link.
(Other definitions for settle that I've seen before include "Pay or come to rest" , "Become established or pay bill" , "Take up residence" , "Agree or decide (one's position)" , "Become calm and quiet" .)