Made up one's mind to be positive (7)
I believe the answer is:
decided
I believe this clue is a double definition.
'made up one's mind' is the first definition.
('decide' can be a synonym of 'make up one's mind')
'positive' is the second definition.
(thesaurus)
'to be' acts as a link.
(Other definitions for decided that I've seen before include "Categorical" , "unmistakable" , "Came to a resolution" , "Determined; resolute" , "Made mind up, being unequivocal" .)