Stupid turning over black wet soil (4)
I believe the answer is:
dumb
'stupid' is the definition.
('dumb' can be similar in meaning to 'stupid')
'turning over black wet soil' is the wordplay.
'turning over' says the letters should be written in reverse.
'black' becomes 'b' (as in pencil classifications).
'wet soil' becomes 'mud' (I've seen this before).
'b'+'mud'='bmud'
'bmud' in reverse letter order is 'DUMB'.
(Other definitions for dumb that I've seen before include "Stupid; unable to speak" , "Stupid (sl.)" , "Lacking speech" , "Lacking intellectual acuity" , "Thick" .)