Dirt, unpleasant, at bottom of scuttle (5)
I believe the answer is:
grime
'dirt' is the definition.
(synonyms)
'unpleasant at bottom of scuttle' is the wordplay.
'unpleasant' becomes 'grim' (similar in meaning).
'at' is a charade indicator (letters next to each other).
'bottom of' says to take the final letters.
The final letter of 'scuttle' is 'e'.
'grim'+'e'='GRIME'
(Other definitions for grime that I've seen before include "Muck" , "Dirt, grease" , "Filth, dirt" , "Ingrained dirt" , "Engrained dirt" .)