Square one jump (5)
I believe the answer is:
start
I believe this is a double definition.
'square one' is the first definition.
(thesaurus)
'jump' is the second definition.
('start' can be a synonym of 'jump')
(Other definitions for start that I've seen before include "Origin" , "Onset" , "Move quickly < when surprised" , "get under way" , "Outset" .)