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