Middle of plot on American soil (4)
I believe the answer is:
loam
'soil' is the definition.
(loam is a kind of soil)
'middle of plot on american' is the wordplay.
'middle of' indicates the central letters.
'on' says to put letters next to each other.
'american' becomes 'am' (abbreviation in some contexts).
The centre of 'plot' is 'lo'.
'lo'+'am'='LOAM'
(Other definitions for loam that I've seen before include "Rich soil, good for growing" , "A rich fertile soil" , "Rich soil containing decayed matter" , "Rich soil with decayed vegetable matter" , "Rich soil consisting of a mixture of sand, clay and decaying organic materials" .)