Resistance unit primarily orders heavy machinery (3)
I believe the answer is:
ohm
'resistance unit' is the definition.
(ohm is the unit of electrical resistance)
'primarily orders heavy machinery' is the wordplay.
'primarily' says to take the initial letters.
The initial letters of 'orders heavy machinery' is 'OHM'.
(Other definitions for ohm that I've seen before include "SI unit of electrical resistance" , "measure of resistance" , "physical unit" , "Physicist" , "lawmaker" .)