Initially provided set up in new start (2,5)
I believe the answer is:
at first
'initially' is the definition.
('at first' can be a synonym of 'initially')
'provided set up in new start' is the wordplay.
'provided' becomes 'if'.
'set up' says the letters should be written backwards.
'in' indicates putting letters inside.
'new' is an anagram indicator.
'if' back-to-front is 'fi'.
'start' is an anagram of 'atrst'.
'fi' placed inside 'atrst' is 'AT FIRST'.
(Other definitions for at first that I've seen before include "In the beginning, to start with" , "Originally" , "To begin with" , "In the beginning, initially" .)