Measurement system encountered resistance with current Celsius (6)
I believe the answer is:
metric
'measurement system' is the definition.
(I've seen this before)
'encountered resistance with current celsius' is the wordplay.
'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').
'met'+'r'+'i'+'c'='METRIC'
(Other definitions for metric that I've seen before include "Relating to poetic rhythm" , "Of modern measuring units" , "Consisting of verses; pertaining to one system of measurement" , "Non-Imperial system of measurement" , "Relating to the standard European system of measurement" .)