Behold a medium soil (4)
I believe the answer is:
loam
'soil' is the definition.
(loam is a kind of soil)
'behold a medium' is the wordplay.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'medium' becomes 'm' (as in UK clothing sizes).
'lo'+'a'+'m'='LOAM'
(Other definitions for loam that I've seen before include "A rich fertile soil" , "Rich soil consisting of a mixture of sand, clay and decaying organic materials" , "Type of soil" , "Rich soil with decayed vegetable matter" , "Rich soil, good for growing" .)