Most of dirt is unpleasant (4)
I believe the answer is:
grim
'unpleasant' is the definition.
(similar in meaning)
'most of dirt' is the wordplay.
'most of' means to remove the last letter.
'dirt' becomes 'grime' (synonyms).
'grime' with its final letter taken off is 'GRIM'.
'is' acts as a link.
(Other definitions for grim that I've seen before include "Very serious/gloomy" , "Tight-lipped and foreboding" , "Severe; horrible" , "(Of humour) black; unappealing, depressing" , "Resolute" .)