A lot of dirt is ghastly (4)
I believe the answer is:
grim
'is ghastly' is the definition.
(I've seen this before)
'a lot of dirt' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'dirt' becomes 'grime' (grime can mean dirt or mud).
'grime' with its final letter taken off is 'GRIM'.
(Other definitions for grim that I've seen before include "Dour and severe" , "Harsh and merciless" , "Dire" , "Very serious, gloomy" , "Dour, forbidding" .)