Note slime fouling up wayside marker (9)
I believe the answer is:
milestone
'wayside marker' is the definition.
(marker at the side of a road)
'note slime fouling up' is the wordplay.
'fouling up' is an anagram indicator.
'note'+'slime'='noteslime'
'noteslime' with letters rearranged gives 'MILESTONE'.
(Other definitions for milestone that I've seen before include "Pillar" , "Significant event or stage - a measure" , "Significant developmental stage" , "Significant stage of development" , "Roadside distance marker" .)