Not more than (4)
'not more than' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mere that I've seen before include "Bare, simple" , "Without additions or modifications" , "Nothing more than is specified - or a poetic lake" , "Lake; Wiltshire town" , "Unimpressive" .)
(Other definitions for merely that I've seen before include "Just one" , "Simply - only" , "'Only, and nothing more (6)'" .)