Gentler (6)
'gentler' is the definition.
('mild' can be similar in meaning to 'gentle')
This is all the clue.
(Another definition for milder that I've seen is " ... more calm".)
(Other definitions for nicer that I've seen before include "Kinder; finer" , "comparatively agreeable" , "it's more appealing" , "More pleasant or courteous" , "Pleasanter" .)