Simply, only (6)
I believe the answer is:
merely
I believe this clue is a double definition.
'simply' is the first definition.
(I know that simply can be written as merely)
'only' is the second definition.
(merely can mean only or simply)
(Other definitions for merely that I've seen before include "Simply - only" , "'Only, and nothing more (6)'" , "Just one" .)