It measures electrical resistance (8)
I believe the answer is:
ohmmeter
'it measures electrical resistance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ohmmeter that I've seen before include "What'll show resistance" , "Measurer of electrical resistance" , "Instrument to measure electrical resistance" , "shows signs of resistance" , "Meter for measuring electrical resistance" .)