Frenchman and Yank upset one managing plots (8)
I believe the answer is:
gardener
'one managing plots' is the definition.
'frenchman and yank upset' is the wordplay.
'frenchman' becomes 'rene'.
'and' says to put letters next to each other.
'yank' becomes 'drag' (I've seen this before).
'upset' says the letters should be written backwards.
'rene'+'drag'='renedrag'
'renedrag' reversed gives 'GARDENER'.
(Other definitions for gardener that I've seen before include "One may get food" , "someone in bed" , "Nursery employee" , "Plant and veg grower" , "Eg, Capability Brown" .)