Landmark developing slime, note (9)
I believe the answer is:
milestone
'landmark' is the definition.
(both can mean a significant moment)
'developing slime note' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'note' becomes 'tone' (synonyms).
'slime' is an anagram of 'miles'.
'miles'+'tone'='MILESTONE'
(Other definitions for milestone that I've seen before include "Pillar" , "Roadside distance marker" , "Significant developmental stage" , "Wayside indicator" , "Important stage (reached)" .)