Decides upon (4)
I believe the answer is:
sets
'decides upon' is the definition.
('set' can be a synonym of 'decide upon')
This is the entire clue.
(Other definitions for sets that I've seen before include "Major divisions of tennis match" , "Scenery etc. of 11 across productions" , "Units of play in tennis" , "Coagulates" , "Plants" .)