Instrument to measure electric current (7)
I believe the answer is:
ammeter
'instrument to measure electric current' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ammeter that I've seen before include "Instrument for measuring electric current" , "Instrument that measures electric current." , "Instrument measuring electrical current" , "Device measuring electric current" , "Current-measurer" .)