Measuring system encountered, endlessly valuable (6)
I believe the answer is:
metric
'measuring system' is the definition.
(I've seen this before)
'encountered endlessly valuable' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'endlessly' means to remove the last letter.
'valuable' becomes 'rich' (similar in meaning).
'rich' with its final letter taken away is 'ric'.
'met'+'ric'='METRIC'
(Other definitions for metric that I've seen before include "Like the SI system" , "Like some feet" , "Quantitative" , "Non-Imperial system of measurement" , "System using litre and gram as units" .)