Swindle over farm machinery one is engaged to supply (10)
I believe the answer is:
contractor
'one is engaged to supply' is the definition.
(a contractor supplies a service)
'swindle over farm machinery' is the wordplay.
'swindle' becomes 'con' (con is a kind of swindle).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'farm machinery' becomes 'tractor'.
'con'+'tractor'='CONTRACTOR'
(Other definitions for contractor that I've seen before include "Businessman" , "One formally agreeing to do job" , "Party to a deal; builder" , "One undertaking specific business" .)