Middle of plot on American soil (4)
I believe the answer is:
Here is my best explanation:
'soil' is the definition.
(loam is a kind of soil)
'middle of plot on american' is the subsidiary indication.
'middle of' means to look at the middle letters.
'on' is a charade indicator (letters next to each other).
'american' becomes 'am' (abbreviation in some contexts).
The middle of 'plot' is 'lo'.
(Other definitions for loam that I've seen before include "Soil", "A rich fertile soil", "Rich soil consisting of a mixture of sand, clay and decaying organic materials", "Fertile soil", "Rich soil with decayed vegetable matter", "Type of soil", "Rich soil, good for growing" and "Rich soil containing decayed matter".)