Begins (5)
'begins' is the definition.
('start' can be a synonym of 'begin')
This is the entire clue.
(Other definitions for starts that I've seen before include "jerks" , "begins or recoils" , "Begins or flinches" , "breaks" , "origins?" .)
'begins' is the definition.
(opening is a kind of beginning)
This is all the clue.
(Other definitions for opens that I've seen before include "Establishes" , "Unseals" , "Launches" , "Exposes, begins" , "Tournaments for both professionals and amateurs" .)