"Not so loud!" (3)
(Other definitions for shh that I've seen before include "Be quiet!" , "Be silent" .)
'not so loud' is the definition.
('soft' can be the opposite of 'loud' so perhaps 'softer' is reasonable for 'not so loud')
This is all the clue.
(Other definitions for softer that I've seen before include "Less hard, more yielding" , "Less firm, loud" , "More pliable or malleable" , "Less hard, more malleable" .)