Electrical unit erected inside farmhouse (3)
I believe the answer is:
ohm
'electrical unit' is the definition.
(ohm is the unit of electrical resistance)
'erected inside farmhouse' is the wordplay.
'erected' shows that the letters should be reversed in order (in a down clue, letters go up).
'inside' indicates a hidden word.
'mho' is hidden in the letters of 'farmhouse'.
'mho' back-to-front is 'OHM'.
(Other definitions for ohm that I've seen before include "SI unit of electrical resistance" , "Physicist" , "measure of resistance" , "A little resistance" , "physical unit" .)