Measurement system encountered resistance with current Celsius (6)
I believe the answer is:
Here is my best explanation:
'measurement system' is the definition.
'encountered resistance with current celsius' is the subsidiary indication.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'resistance' becomes 'r' (abbreviation).
'with' is a charade indicator (letters next to each other).
'current' becomes 'i' (symbol for electric current).
'celsius' becomes 'c' ('c' can be a synonym of 'celsius').
(Other definitions for metric that I've seen before include "Consisting of verses; pertaining to one system of measurement", "Relating to poetic rhythm", "System of measurement", "Like decimal measuring system", "System of weights and measures", "Relating to the standard European system of measurement", "Measuring system", "Of a system of measurement", "Decimal" and "Like the SI system".)