In the first place, mainly (9)
I believe the answer is:
primarily
I believe this clue is a double definition.
'in the first place' is the first definition.
(synonyms)
'mainly' is the second definition.
(I know that mainly can be written as primarily)
(Other definitions for primarily that I've seen before include "Chiefly, principally" , "Firstly" , "Basically" , "Chiefly, in the first place" , "In the first place, mainly" .)